Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

38 files changed, 9287 insertions, 0 deletions

diff --git a/layout/xul/tree/crashtests/307298-1.xhtml b/layout/xul/tree/crashtests/307298-1.xhtml
new file mode 100644
index 0000000000..6c04a01321
--- /dev/null
+++ b/layout/xul/tree/crashtests/307298-1.xhtml
@@ -0,0 +1,21 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" onload="var tree = document.getElementById('tree'), treeitem = document.getElementById('treeitem'); tree.parentNode.insertBefore(treeitem, tree);">
+
+<tree flex="1" id="tree">
+  <treecols>
+    <treecol id="name" label="Name" primary="true" flex="1"/>
+  </treecols>
+
+  <treechildren>
+    <treeitem id="treeitem">
+      <treerow>
+        <treecell label="Click the button below to crash"/>
+      </treerow>
+    </treeitem>
+  </treechildren>
+</tree>
+
+</window>
diff --git a/layout/xul/tree/crashtests/309732-1.xhtml b/layout/xul/tree/crashtests/309732-1.xhtml
new file mode 100644
index 0000000000..a7e40b75b9
--- /dev/null
+++ b/layout/xul/tree/crashtests/309732-1.xhtml
@@ -0,0 +1,30 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" class="reftest-wait" onload="setTimeout(boom, 30)">
+
+
+  <script>
+  function boom()
+  {
+    document.documentElement.appendChild(document.getElementById("TC")); 
+    document.documentElement.appendChild(document.getElementById("TI"));
+
+    document.documentElement.removeAttribute("class");
+  }
+  </script>
+
+<tree flex="1">
+    <treecols>
+      <treecol label="Name"/>
+    </treecols>
+    <treechildren id="TC">
+      <treeitem id="TI">
+        <treerow>
+          <treecell label="First treecell"/>
+        </treerow>
+      </treeitem>
+    </treechildren>
+  </tree>
+</window>
diff --git a/layout/xul/tree/crashtests/309732-2.xhtml b/layout/xul/tree/crashtests/309732-2.xhtml
new file mode 100644
index 0000000000..354c58dacf
--- /dev/null
+++ b/layout/xul/tree/crashtests/309732-2.xhtml
@@ -0,0 +1,31 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" class="reftest-wait" onload="setTimeout(boom, 30)">
+
+  <script>
+  function boom()
+  {
+    document.documentElement.appendChild(document.getElementById('TC'));   
+    document.getElementById('TI').hidden = false;
+    
+    document.documentElement.removeAttribute("class");
+  }
+  </script>
+
+
+  <tree flex="1">
+    <treecols>
+      <treecol label="Name" flex="1"/>
+    </treecols>
+    <treechildren id="TC">
+      <treeitem>
+        <treerow>
+          <treecell label="First treecell"/>
+        </treerow>
+      </treeitem>
+    </treechildren>
+  </tree>
+  <treeitem id="TI" hidden="true"/>
+</window>
diff --git a/layout/xul/tree/crashtests/366583-1.xhtml b/layout/xul/tree/crashtests/366583-1.xhtml
new file mode 100644
index 0000000000..fd12709905
--- /dev/null
+++ b/layout/xul/tree/crashtests/366583-1.xhtml
@@ -0,0 +1,43 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="boom1();"
+        class="reftest-wait">
+
+<script>
+        
+var tree;
+
+function boom1()
+{
+  tree = document.getElementById("tree");
+  tree.style.position = "fixed";
+  setTimeout(boom2, 30);
+}
+
+function boom2()
+{
+  tree.style.overflow = "visible";
+  document.documentElement.removeAttribute("class");
+}
+</script>
+
+<tree rows="6" id="tree" style="display: list-item; overflow: auto; visibility: collapse;">
+  <treecols>
+      <treecol id="firstname" label="First Name" primary="true" style="-moz-box-flex: 3"/>
+      <treecol id="lastname" label="Last Name" style="-moz-box-flex: 7"/>
+  </treecols>
+
+  <treechildren>
+    <treeitem container="true" open="true">
+      <treerow>
+        <treecell label="Foo"/>
+      </treerow>
+    </treeitem>
+  </treechildren>
+</tree>
+
+
+</window>
diff --git a/layout/xul/tree/crashtests/380217-1.xhtml b/layout/xul/tree/crashtests/380217-1.xhtml
new file mode 100644
index 0000000000..251b3c450d
--- /dev/null
+++ b/layout/xul/tree/crashtests/380217-1.xhtml
@@ -0,0 +1,31 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+<window xmlns:html="http://www.w3.org/1999/xhtml"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="document.documentElement.style.content = '\'a\'';">
+
+<html:style>
+* { position: fixed; }
+*:not(style) {
+  /* At the time this testcase was added, the above `float` styling would
+     have automatically forced "display:block" for these elements, so we
+     should preserve that styling to preserve the integrity of the crashtest
+     since blockification behavior for -moz-box is changing. */
+  display: block;
+}
+</html:style>
+
+<tree rows="6">
+  <treecols>
+      <treecol id="firstname" label="First Name" primary="true"/>
+  </treecols>
+  <treechildren>
+    <treeitem>
+      <treerow>
+        <treecell/>
+      </treerow>
+    </treeitem>
+  </treechildren>
+</tree>
+
+</window>
diff --git a/layout/xul/tree/crashtests/382444-1-inner.html b/layout/xul/tree/crashtests/382444-1-inner.html
new file mode 100644
index 0000000000..01805e6b34
--- /dev/null
+++ b/layout/xul/tree/crashtests/382444-1-inner.html
@@ -0,0 +1,15 @@
+<html>

+<head>

+<title>Testcase bug - Crash [@ nsINodeInfo::Equals] with underflow event, tree stuff and removing window</title>

+</head>

+<body>

+<iframe src="data:application/xhtml+xml;charset=utf-8,%3Cwindow%20xmlns%3D%22http%3A//www.mozilla.org/keymaster/gatekeeper/there.is.only.xul%22%3E%0A%3Ctree%20style%3D%22overflow%3A%20auto%3B%20display%3A%20-moz-inline-box%3B%22%3E%0A%3Ctreeitem%20style%3D%22overflow%3A%20scroll%3B%20display%3A%20table-cell%3B%22%3E%0A%3Ctreechildren%20style%3D%22%20display%3A%20table-row%3B%22%3E%0A%3Ctreeitem%20id%3D%22a%22%20style%3D%22display%3A%20table-cell%3B%22%3E%0A%3C/treeitem%3E%0A%3C/treechildren%3E%0A%3C/treeitem%3E%0A%0A%3C/tree%3E%0A%0A%3Cscript%20xmlns%3D%22http%3A//www.w3.org/1999/xhtml%22%3E%0Afunction%20doe%28%29%20%7B%0Adocument.getElementById%28%27a%27%29.parentNode.removeChild%28document.getElementById%28%27a%27%29%29%3B%0A%7D%0AsetTimeout%28doe%2C%20100%29%3B%0Adocument.addEventListener%28%27underflow%27%2C%20function%28e%29%20%7Bwindow.frameElement.parentNode.removeChild%28window.frameElement%29%20%7D%2C%20true%29%3B%0Awindow.addEventListener%28%27underflow%27%2C%20function%28e%29%20%7Bwindow.frameElement.parentNode.removeChild%28window.frameElement%29%20%7D%2C%20true%29%3B%0A%3C/script%3E%0A%3C/window%3E" id="content"></iframe>

+

+<script>

+function doe() {

+window.location.reload();

+}

+setTimeout(doe, 500);

+</script>

+</body>

+</html>

diff --git a/layout/xul/tree/crashtests/382444-1.html b/layout/xul/tree/crashtests/382444-1.html
new file mode 100644
index 0000000000..8926cf16d7
--- /dev/null
+++ b/layout/xul/tree/crashtests/382444-1.html
@@ -0,0 +1,9 @@
+<html class="reftest-wait">
+<head>
+<script>
+setTimeout('document.documentElement.className = ""', 500);
+</script>
+<body>
+<iframe src="382444-1-inner.html"></iframe>
+</body>
+</html>
diff --git a/layout/xul/tree/crashtests/391178-1.xhtml b/layout/xul/tree/crashtests/391178-1.xhtml
new file mode 100644
index 0000000000..0f4b16cd99
--- /dev/null
+++ b/layout/xul/tree/crashtests/391178-1.xhtml
@@ -0,0 +1,41 @@
+<html xmlns="http://www.w3.org/1999/xhtml" class="reftest-wait">
+<head>
+<script>
+
+var ccc;
+
+function boom()
+{
+  var XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+  var hbox = document.createElementNS(XUL_NS, 'hbox');
+  var tree = document.createElementNS(XUL_NS, 'tree');
+  var treecol = document.createElementNS(XUL_NS, 'treecol');
+
+  ccc = document.getElementById("ccc");
+
+  ccc.style.position = "fixed";
+
+  hbox.appendChild(treecol);
+  tree.appendChild(hbox);
+  ccc.appendChild(tree);
+  
+  setTimeout(boom2, 200);
+}
+
+function boom2()
+{
+  ccc.style.position = "";
+  document.documentElement.removeAttribute("class");
+}
+
+</script>
+</head>
+
+<body onload="boom();">
+
+<div id="ccc">
+</div>
+
+</body>
+</html>
diff --git a/layout/xul/tree/crashtests/391178-2.xhtml b/layout/xul/tree/crashtests/391178-2.xhtml
new file mode 100644
index 0000000000..423b5d1bfe
--- /dev/null
+++ b/layout/xul/tree/crashtests/391178-2.xhtml
@@ -0,0 +1,20 @@
+<?xml-stylesheet href="chrome://browser/skin/" type="text/css"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" xmlns:html="http://www.w3.org/1999/xhtml" class="reftest-wait">
+
+<tree id="a" style="display: block; position: fixed;">
+  <box style=" display: block; position: fixed;">
+    <treecol style=" display: -moz-box;"/>
+  </box>
+  <box style="display: block; position: fixed;">
+    <treechildren style="display: block; position: absolute;"/>
+  </box>
+</tree>
+
+<script xmlns="http://www.w3.org/1999/xhtml">
+function removestyles(){
+  document.getElementById('a').removeAttribute('style');
+  document.documentElement.removeAttribute("class");
+}
+setTimeout(removestyles, 100);
+</script>
+</window>
diff --git a/layout/xul/tree/crashtests/393665-1.xhtml b/layout/xul/tree/crashtests/393665-1.xhtml
new file mode 100644
index 0000000000..6fb5ec0c9e
--- /dev/null
+++ b/layout/xul/tree/crashtests/393665-1.xhtml
@@ -0,0 +1,3 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <treechildren style="display: block" />
+</window>
diff --git a/layout/xul/tree/crashtests/399227-1.xhtml b/layout/xul/tree/crashtests/399227-1.xhtml
new file mode 100644
index 0000000000..3ae4dfa764
--- /dev/null
+++ b/layout/xul/tree/crashtests/399227-1.xhtml
@@ -0,0 +1,44 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" class="reftest-wait" onload="setTimeout(boom, 30)">
+
+
+  <script>
+  function boom()
+  {
+    var tree = document.getElementById("thetree");
+    var selection = tree.view.selection;
+
+    selection.select(0);
+    tree.parentNode.removeChild(tree);
+
+    // This is expected to throw an error (it used to crash).
+    try {
+      selection.rangedSelect(1, 1, false);
+    }
+    catch (ex) {}
+
+    document.documentElement.removeAttribute("class");
+  }
+  </script>
+
+<tree flex="1" id="thetree">
+    <treecols>
+      <treecol label="Name"/>
+    </treecols>
+    <treechildren id="TC">
+      <treeitem id="TI1">
+        <treerow>
+          <treecell label="First treecell"/>
+        </treerow>
+      </treeitem>
+      <treeitem id="TI2">
+        <treerow>
+          <treecell label="Second treecell"/>
+        </treerow>
+      </treeitem>
+    </treechildren>
+  </tree>
+</window>
diff --git a/layout/xul/tree/crashtests/399692-1.xhtml b/layout/xul/tree/crashtests/399692-1.xhtml
new file mode 100644
index 0000000000..97eec26742
--- /dev/null
+++ b/layout/xul/tree/crashtests/399692-1.xhtml
@@ -0,0 +1,10 @@
+<html xmlns="http://www.w3.org/1999/xhtml"
+      xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<head>
+</head>
+<body>
+
+<xul:treechildren style="display: inline;" />
+
+</body>
+</html>
diff --git a/layout/xul/tree/crashtests/399715-1.xhtml b/layout/xul/tree/crashtests/399715-1.xhtml
new file mode 100644
index 0000000000..ea0a20cfa2
--- /dev/null
+++ b/layout/xul/tree/crashtests/399715-1.xhtml
@@ -0,0 +1,9 @@
+<html xmlns="http://www.w3.org/1999/xhtml"
+      xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+      
+<body style="float: right;" onload="document.body.style.cssFloat = '';">
+
+<xul:tree><xul:hbox><xul:treecol /></xul:hbox></xul:tree>
+
+</body>
+</html>
diff --git a/layout/xul/tree/crashtests/409807-1.xhtml b/layout/xul/tree/crashtests/409807-1.xhtml
new file mode 100644
index 0000000000..a3af3da41b
--- /dev/null
+++ b/layout/xul/tree/crashtests/409807-1.xhtml
@@ -0,0 +1,25 @@
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" onload="boom();">
+
+<script type="text/javascript">
+
+function boom()
+{
+  var tree = document.getElementById("tree");
+  var tc = document.createElementNS("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "treechildren");
+  
+  document.addEventListener("DOMAttrModified", m, false);
+  
+  tree.appendChild(tc);
+  
+  function m()
+  {
+    document.removeEventListener("DOMAttrModified", m, false);
+    tree.removeChild(tc);
+  }
+}
+
+</script>
+
+<tree id="tree" />
+
+</window>
diff --git a/layout/xul/tree/crashtests/414170-1.xhtml b/layout/xul/tree/crashtests/414170-1.xhtml
new file mode 100644
index 0000000000..82ea63bcfd
--- /dev/null
+++ b/layout/xul/tree/crashtests/414170-1.xhtml
@@ -0,0 +1,20 @@
+<?xml version="1.0"?>
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="boom();">
+        
+<script type="text/javascript">
+
+function boom()
+{
+  var option = document.createElementNS("http://www.w3.org/1999/xhtml", "option");
+  document.getElementById("tc").appendChild(option);
+}
+
+</script>
+
+<tree><treechildren id="tc"><hbox/></treechildren></tree>
+
+</window>
diff --git a/layout/xul/tree/crashtests/479931-1.xhtml b/layout/xul/tree/crashtests/479931-1.xhtml
new file mode 100644
index 0000000000..458a192501
--- /dev/null
+++ b/layout/xul/tree/crashtests/479931-1.xhtml
@@ -0,0 +1,19 @@
+<html xmlns="http://www.w3.org/1999/xhtml" xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<head>
+<script type="text/javascript">
+
+function boom()
+{
+  var o = document.createElementNS("http://www.w3.org/1999/xhtml", "option");
+  var q = document.getElementById("q");
+  q.appendChild(o);
+}
+
+</script>
+</head>
+<body onload="boom();">
+
+<xul:tree><xul:treechildren id="q"><div/></xul:treechildren></xul:tree>
+
+</body>
+</html>
diff --git a/layout/xul/tree/crashtests/585815-iframe.xhtml b/layout/xul/tree/crashtests/585815-iframe.xhtml
new file mode 100644
index 0000000000..90c20fca80
--- /dev/null
+++ b/layout/xul/tree/crashtests/585815-iframe.xhtml
@@ -0,0 +1,72 @@
+<?xml version="1.0"?>
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="setInterval(run, 25)">
+
+<tree style="-moz-box-flex: 1" rows="2">
+ <treecols>  
+    <treecol id="sender" label="Sender" style="-moz-box-flex: 1"/>  
+    <treecol id="subject" label="Subject" style="-moz-box-flex: 2"/>  
+  </treecols>  
+  <treechildren>
+    <treeitem>
+      <treerow>
+        <treecell label="joe@somewhere.com"/>  
+        <treecell label="Top secret plans"/>  
+      </treerow>
+    </treeitem>  
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+    <treeitem>  
+      <treerow>  
+        <treecell label="mel@whereever.com"/>  
+        <treecell label="Let's do lunch"/>  
+      </treerow>  
+    </treeitem> 
+  </treechildren>
+</tree>
+
+<script type="text/javascript"><![CDATA[
+function run() {
+  var tree = document.getElementsByTagName("tree")[0];
+  var sel = tree.view.selection;
+  sel.rangedSelect(0, 0, true);
+  sel.rangedSelect(1000, 1001, true);
+  sel.adjustSelection(1, 0x7fffffff);
+}
+]]></script>
+
+</window>
diff --git a/layout/xul/tree/crashtests/585815.html b/layout/xul/tree/crashtests/585815.html
new file mode 100644
index 0000000000..7c3b27f6aa
--- /dev/null
+++ b/layout/xul/tree/crashtests/585815.html
@@ -0,0 +1,18 @@
+<!DOCTYPE HTML>
+<html class="reftest-wait"><head>
+    <meta charset="utf-8">
+    <title>Testcase for bug 585815</title>
+<script>
+function done()
+{
+  document.documentElement.removeAttribute("class");
+}
+</script>
+</head>
+<body onload="setTimeout(done,1000)">
+
+<iframe src="585815-iframe.xhtml"></iframe>
+
+
+</body>
+</html>
diff --git a/layout/xul/tree/crashtests/601427.html b/layout/xul/tree/crashtests/601427.html
new file mode 100644
index 0000000000..2a2999052e
--- /dev/null
+++ b/layout/xul/tree/crashtests/601427.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<script>
+
+var onPaintFunctions =
+[
+  function() { document.documentElement.style.MozAppearance = "treeheadersortarrow"; },
+  function() { document.documentElement.style.position = "fixed"; },
+  function() { document.documentElement.removeAttribute("class"); }
+];
+
+var i = 0;
+
+function advance()
+{
+  var f = onPaintFunctions[i++];
+  if (f)
+    f();
+}
+
+function start()
+{
+  window.addEventListener("MozAfterPaint", advance, true);
+  advance();
+}
+
+window.addEventListener("load", start);
+
+</script>
+</html>
diff --git a/layout/xul/tree/crashtests/730441-3.xhtml b/layout/xul/tree/crashtests/730441-3.xhtml
new file mode 100644
index 0000000000..c3fe199a83
--- /dev/null
+++ b/layout/xul/tree/crashtests/730441-3.xhtml
@@ -0,0 +1,38 @@
+<?xml version="1.0"?>
+<!--
+###!!! ASSERTION: You can't dereference a NULL nsCOMPtr with operator->().: 'mRawPtr != 0', file ../../../../dist/include/nsCOMPtr.h, line 796
+
+Program received signal SIGSEGV, Segmentation fault.
+0xb6b7463a in nsTreeContentView::SetTree (this=0xb0ba2510, aTree=0xaaecece0) at layout/xul/base/src/tree/src/nsTreeContentView.cpp:571
+571        boxObject->GetElement(getter_AddRefs(element));
+(gdb) bt 3
+#0  0xb6b7463a in nsTreeContentView::SetTree (this=0xb0ba2510, aTree=0xaaecece0) at layout/xul/base/src/tree/src/nsTreeContentView.cpp:571
+#1  0xb736c76f in NS_InvokeByIndex_P () at xpcom/reflect/xptcall/md/unix/xptcinvoke_gcc_x86_unix.cpp:69
+#2  0xb6171901 in XPCWrappedNative::CallMethod (ccx=..., mode=XPCWrappedNative::CALL_METHOD)
+    at js/src/xpconnect/src/xpcwrappednative.cpp:2722
+(More stack frames follow...)
+(gdb) list 566
+561    nsTreeContentView::SetTree(nsITreeBoxObject* aTree)
+562    {
+563      ClearRows();
+564
+565      mBoxObject = aTree;
+566
+567      if (aTree && !mRoot) {
+568        // Get our root element
+569        nsCOMPtr<nsIBoxObject> boxObject = do_QueryInterface(mBoxObject);
+570        nsCOMPtr<Element> element;
+571        boxObject->GetElement(getter_AddRefs(element));
+(gdb) p boxObject
+$16 = {mRawPtr = 0x0}
+
+|aTree| does not implement |nsIBoxObject|, so |do_QueryInterface(mBoxObject)|
+returns null. Then we have |null->GetElement()|.
+-->
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="document.getElementById('tree').view.setTree({});">
+<tree id="tree">
+  <treechildren/>
+</tree>
+</window>
+
diff --git a/layout/xul/tree/crashtests/crashtests.list b/layout/xul/tree/crashtests/crashtests.list
new file mode 100644
index 0000000000..81f6ab7e6c
--- /dev/null
+++ b/layout/xul/tree/crashtests/crashtests.list
@@ -0,0 +1,18 @@
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/307298-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/309732-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/309732-2.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/366583-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/380217-1.xhtml
+load 382444-1.html
+load 391178-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/391178-2.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/393665-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/399227-1.xhtml
+load 399692-1.xhtml
+load 399715-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/409807-1.xhtml
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/414170-1.xhtml
+load 479931-1.xhtml
+load 585815.html
+pref(widget.windows.window_occlusion_tracking.enabled,false) load 601427.html # Bug 1819154
+load chrome://reftest/content/crashtests/layout/xul/tree/crashtests/730441-3.xhtml
diff --git a/layout/xul/tree/moz.build b/layout/xul/tree/moz.build
new file mode 100644
index 0000000000..f99b7bd41c
--- /dev/null
+++ b/layout/xul/tree/moz.build
@@ -0,0 +1,44 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Core", "XUL")
+
+XPIDL_SOURCES += [
+    "nsITreeSelection.idl",
+    "nsITreeView.idl",
+]
+
+XPIDL_MODULE = "layout_xul_tree"
+
+EXPORTS += [
+    "nsTreeColumns.h",
+    "nsTreeUtils.h",
+]
+
+UNIFIED_SOURCES += [
+    "nsTreeBodyFrame.cpp",
+    "nsTreeColumns.cpp",
+    "nsTreeContentView.cpp",
+    "nsTreeImageListener.cpp",
+    "nsTreeSelection.cpp",
+    "nsTreeStyleCache.cpp",
+    "nsTreeUtils.cpp",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
+LOCAL_INCLUDES += [
+    "..",
+    "../../base",
+    "../../forms",
+    "../../generic",
+    "../../painting",
+    "../../style",
+    "/dom/base",
+    "/dom/xul",
+]
diff --git a/layout/xul/tree/nsITreeSelection.idl b/layout/xul/tree/nsITreeSelection.idl
new file mode 100644
index 0000000000..d265b639ee
--- /dev/null
+++ b/layout/xul/tree/nsITreeSelection.idl
@@ -0,0 +1,119 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+webidl XULTreeElement;
+
+[scriptable, uuid(ab6fe746-300b-4ab4-abb9-1c0e3977874c)]
+interface nsITreeSelection : nsISupports
+{
+  /**
+   * The tree widget for this selection.
+   */
+  attribute XULTreeElement tree;
+
+  /**
+   * This attribute is a boolean indicating single selection.
+   */
+  readonly attribute boolean single;
+
+  /**
+   * The number of rows currently selected in this tree.
+   */
+  readonly attribute long count;
+
+  /**
+   * Indicates whether or not the row at the specified index is
+   * part of the selection.
+   */
+  boolean isSelected(in long index);
+
+  /**
+   * Deselect all rows and select the row at the specified index.
+   */
+  void select(in long index);
+
+  /**
+   * Perform a timed select.
+   */
+  void timedSelect(in long index, in long delay);
+
+  /**
+   * Toggle the selection state of the row at the specified index.
+   */
+  void toggleSelect(in long index);
+
+  /**
+   * Select the range specified by the indices.  If augment is true,
+   * then we add the range to the selection without clearing out anything
+   * else.  If augment is false, everything is cleared except for the specified range.
+   */
+  void rangedSelect(in long startIndex, in long endIndex, in boolean augment);
+
+  /**
+   * Clears the range.
+   */
+  void clearRange(in long startIndex, in long endIndex);
+
+  /**
+   * Clears the selection.
+   */
+  void clearSelection();
+
+  /**
+   * Selects all rows.
+   */
+  void selectAll();
+
+  /**
+   * Iterate the selection using these methods.
+   */
+  long getRangeCount();
+  void getRangeAt(in long i, out long min, out long max);
+
+  /**
+   * Can be used to invalidate the selection.
+   */
+  void invalidateSelection();
+
+  /**
+   * Called when the row count changes to adjust selection indices.
+   */
+  void adjustSelection(in long index, in long count);
+
+  /**
+   * This attribute is a boolean indicating whether or not the
+   * "select" event should fire when the selection is changed using
+   * one of our methods.  A view can use this to temporarily suppress
+   * the selection while manipulating all of the indices, e.g., on
+   * a sort.
+   * Note: setting this attribute to false will fire a select event.
+   */
+  attribute boolean selectEventsSuppressed;
+
+  /**
+   * The current item (the one that gets a focus rect in addition to being
+   * selected).
+   */
+  attribute long currentIndex;
+
+  /**
+   * The selection "pivot".  This is the first item the user selected as
+   * part of a ranged select.
+   */
+  readonly attribute long shiftSelectPivot;
+};
+
+/**
+ * The following interface is not scriptable and MUST NEVER BE MADE scriptable.
+ * Native treeselections implement it, and we use this to check whether a
+ * treeselection is native (and therefore suitable for use by untrusted content).
+ */
+[uuid(1bd59678-5cb3-4316-b246-31a91b19aabe)]
+interface nsINativeTreeSelection : nsITreeSelection
+{
+  [noscript] void ensureNative();
+};
diff --git a/layout/xul/tree/nsITreeView.idl b/layout/xul/tree/nsITreeView.idl
new file mode 100644
index 0000000000..7f2480ceaf
--- /dev/null
+++ b/layout/xul/tree/nsITreeView.idl
@@ -0,0 +1,173 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsITreeSelection;
+
+webidl DataTransfer;
+webidl TreeColumn;
+webidl XULTreeElement;
+
+[scriptable, uuid(091116f0-0bdc-4b32-b9c8-c8d5a37cb088)]
+interface nsITreeView : nsISupports
+{
+  /**
+   * The total number of rows in the tree (including the offscreen rows).
+   */
+  readonly attribute long rowCount;
+
+  /**
+   * The selection for this view.
+   */
+  attribute nsITreeSelection selection;
+
+  /**
+   * A whitespace delimited list of properties.  For each property X the view
+   * gives back will cause the pseudoclasses  ::-moz-tree-cell(x),
+   * ::-moz-tree-row(x), ::-moz-tree-twisty(x), ::-moz-tree-image(x),
+   * ::-moz-tree-cell-text(x).  to be matched on the pseudoelement
+   * ::moz-tree-row.
+   */
+  AString getRowProperties(in long index);
+
+  /**
+   * A whitespace delimited list of properties for a given cell.  Each
+   * property, x, that the view gives back will cause the pseudoclasses
+   *  ::-moz-tree-cell(x), ::-moz-tree-row(x), ::-moz-tree-twisty(x),
+   *  ::-moz-tree-image(x), ::-moz-tree-cell-text(x). to be matched on the
+   *  cell.
+   */
+  AString getCellProperties(in long row, in TreeColumn col);
+
+  /**
+   * Called to get properties to paint a column background.  For shading the sort
+   * column, etc.
+   */
+  AString getColumnProperties(in TreeColumn col);
+
+  /**
+   * Methods that can be used to test whether or not a twisty should be drawn,
+   * and if so, whether an open or closed twisty should be used.
+   */
+  boolean isContainer(in long index);
+  boolean isContainerOpen(in long index);
+  boolean isContainerEmpty(in long index);
+
+  /**
+   * isSeparator is used to determine if the row at index is a separator.
+   * A value of true will result in the tree drawing a horizontal separator.
+   * The tree uses the ::moz-tree-separator pseudoclass to draw the separator.
+   */
+  boolean isSeparator(in long index);
+
+  /**
+   * Specifies if there is currently a sort on any column. Used mostly by dragdrop
+   * to affect drop feedback.
+   */
+  boolean isSorted();
+
+  const short DROP_BEFORE = -1;
+  const short DROP_ON = 0;
+  const short DROP_AFTER = 1;
+  /**
+   * Methods used by the drag feedback code to determine if a drag is allowable at
+   * the current location. To get the behavior where drops are only allowed on
+   * items, such as the mailNews folder pane, always return false when
+   * the orientation is not DROP_ON.
+   */
+  boolean canDrop(in long index, in long orientation, in DataTransfer dataTransfer);
+
+  /**
+   * Called when the user drops something on this view. The |orientation| param
+   * specifies before/on/after the given |row|.
+   */
+  void drop(in long row, in long orientation, in DataTransfer dataTransfer);
+
+  /**
+   * Methods used by the tree to draw thread lines in the tree.
+   * getParentIndex is used to obtain the index of a parent row.
+   * If there is no parent row, getParentIndex returns -1.
+   */
+  long getParentIndex(in long rowIndex);
+
+  /**
+   * hasNextSibling is used to determine if the row at rowIndex has a nextSibling
+   * that occurs *after* the index specified by afterIndex.  Code that is forced
+   * to march down the view looking at levels can optimize the march by starting
+   * at afterIndex+1.
+   */
+  boolean hasNextSibling(in long rowIndex, in long afterIndex);
+
+  /**
+   * The level is an integer value that represents
+   * the level of indentation.  It is multiplied by the width specified in the
+   * :moz-tree-indentation pseudoelement to compute the exact indendation.
+   */
+  long getLevel(in long index);
+
+  /**
+   * The image path for a given cell. For defining an icon for a cell.
+   * If the empty string is returned, the :moz-tree-image pseudoelement
+   * will be used.
+   */
+  AString getImageSrc(in long row, in TreeColumn col);
+
+  /**
+   * The value for a given cell. This method is only called for columns
+   * of type other than |text|.
+   */
+  AString getCellValue(in long row, in TreeColumn col);
+
+  /**
+   * The text for a given cell.  If a column consists only of an image, then
+   * the empty string is returned.
+   */
+  AString getCellText(in long row, in TreeColumn col);
+
+  /**
+   * Called during initialization to link the view to the front end box object.
+   */
+  void setTree(in XULTreeElement tree);
+
+  /**
+   * Called on the view when an item is opened or closed.
+   */
+  void toggleOpenState(in long index);
+
+  /**
+   * Called on the view when a header is clicked.
+   */
+  void cycleHeader(in TreeColumn col);
+
+  /**
+   * Should be called from a XUL onselect handler whenever the selection changes.
+   */
+  [binaryname(SelectionChangedXPCOM)]
+  void selectionChanged();
+
+  /**
+   * Called on the view when a cell in a non-selectable cycling column (e.g., unread/flag/etc.) is clicked.
+   */
+  void cycleCell(in long row, in TreeColumn col);
+
+  /**
+   * isEditable is called to ask the view if the cell contents are editable.
+   * A value of true will result in the tree popping up a text field when
+   * the user tries to inline edit the cell.
+   */
+  boolean isEditable(in long row, in TreeColumn col);
+
+  /**
+   * setCellValue is called when the value of the cell has been set by the user.
+   * This method is only called for columns of type other than |text|.
+   */
+  void setCellValue(in long row, in TreeColumn col, in AString value);
+
+  /**
+   * setCellText is called when the contents of the cell have been edited by the user.
+   */
+  void setCellText(in long row, in TreeColumn col, in AString value);
+};
diff --git a/layout/xul/tree/nsTreeBodyFrame.cpp b/layout/xul/tree/nsTreeBodyFrame.cpp
new file mode 100644
index 0000000000..c7e8272aa0
--- /dev/null
+++ b/layout/xul/tree/nsTreeBodyFrame.cpp
@@ -0,0 +1,4363 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SimpleXULLeafFrame.h"
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/ContentEvents.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/gfx/2D.h"
+#include "mozilla/gfx/PathHelpers.h"
+#include "mozilla/Likely.h"
+#include "mozilla/LookAndFeel.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/MouseEvents.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/intl/Segmenter.h"
+
+#include "gfxUtils.h"
+#include "nsAlgorithm.h"
+#include "nsCOMPtr.h"
+#include "nsComponentManagerUtils.h"
+#include "nsFontMetrics.h"
+#include "nsITreeView.h"
+#include "nsPresContext.h"
+#include "nsNameSpaceManager.h"
+
+#include "nsTreeBodyFrame.h"
+#include "nsTreeSelection.h"
+#include "nsTreeImageListener.h"
+
+#include "nsGkAtoms.h"
+#include "nsCSSAnonBoxes.h"
+
+#include "gfxContext.h"
+#include "nsIContent.h"
+#include "mozilla/ComputedStyle.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/ReferrerInfo.h"
+#include "mozilla/intl/Segmenter.h"
+#include "nsCSSRendering.h"
+#include "nsString.h"
+#include "nsContainerFrame.h"
+#include "nsView.h"
+#include "nsViewManager.h"
+#include "nsVariant.h"
+#include "nsWidgetsCID.h"
+#include "nsIFrameInlines.h"
+#include "nsTreeContentView.h"
+#include "nsTreeUtils.h"
+#include "nsStyleConsts.h"
+#include "nsITheme.h"
+#include "imgIRequest.h"
+#include "imgIContainer.h"
+#include "mozilla/dom/NodeInfo.h"
+#include "nsContentUtils.h"
+#include "nsLayoutUtils.h"
+#include "nsIScrollableFrame.h"
+#include "nsDisplayList.h"
+#include "mozilla/dom/CustomEvent.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/ToJSValue.h"
+#include "mozilla/dom/TreeColumnBinding.h"
+#include <algorithm>
+#include "ScrollbarActivity.h"
+
+#ifdef ACCESSIBILITY
+#  include "nsAccessibilityService.h"
+#  include "nsIWritablePropertyBag2.h"
+#endif
+#include "nsBidiUtils.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+using namespace mozilla::gfx;
+using namespace mozilla::image;
+using namespace mozilla::layout;
+
+enum CroppingStyle { CropNone, CropLeft, CropRight, CropCenter, CropAuto };
+
+// FIXME: Maybe unify with MiddleCroppingBlockFrame?
+static void CropStringForWidth(nsAString& aText, gfxContext& aRenderingContext,
+                               nsFontMetrics& aFontMetrics, nscoord aWidth,
+                               CroppingStyle aCropType) {
+  DrawTarget* drawTarget = aRenderingContext.GetDrawTarget();
+
+  // See if the width is even smaller than the ellipsis
+  // If so, clear the text completely.
+  const nsDependentString& kEllipsis = nsContentUtils::GetLocalizedEllipsis();
+  aFontMetrics.SetTextRunRTL(false);
+  nscoord ellipsisWidth =
+      nsLayoutUtils::AppUnitWidthOfString(kEllipsis, aFontMetrics, drawTarget);
+
+  if (ellipsisWidth > aWidth) {
+    aText.Truncate(0);
+    return;
+  }
+  if (ellipsisWidth == aWidth) {
+    aText.Assign(kEllipsis);
+    return;
+  }
+
+  // We will be drawing an ellipsis, thank you very much.
+  // Subtract out the required width of the ellipsis.
+  // This is the total remaining width we have to play with.
+  aWidth -= ellipsisWidth;
+
+  using mozilla::intl::GraphemeClusterBreakIteratorUtf16;
+  using mozilla::intl::GraphemeClusterBreakReverseIteratorUtf16;
+
+  // Now we crop. This is quite basic: it will not be really accurate in the
+  // presence of complex scripts with contextual shaping, etc., as it measures
+  // each grapheme cluster in isolation, not in its proper context.
+  switch (aCropType) {
+    case CropAuto:
+    case CropNone:
+    case CropRight: {
+      const Span text(aText);
+      GraphemeClusterBreakIteratorUtf16 iter(text);
+      uint32_t pos = 0;
+      nscoord totalWidth = 0;
+
+      while (Maybe<uint32_t> nextPos = iter.Next()) {
+        const nscoord charWidth = nsLayoutUtils::AppUnitWidthOfString(
+            text.FromTo(pos, *nextPos), aFontMetrics, drawTarget);
+        if (totalWidth + charWidth > aWidth) {
+          break;
+        }
+        pos = *nextPos;
+        totalWidth += charWidth;
+      }
+
+      if (pos < aText.Length()) {
+        aText.Replace(pos, aText.Length() - pos, kEllipsis);
+      }
+    } break;
+
+    case CropLeft: {
+      const Span text(aText);
+      GraphemeClusterBreakReverseIteratorUtf16 iter(text);
+      uint32_t pos = text.Length();
+      nscoord totalWidth = 0;
+
+      // nextPos is decreasing since we use a reverse iterator.
+      while (Maybe<uint32_t> nextPos = iter.Next()) {
+        const nscoord charWidth = nsLayoutUtils::AppUnitWidthOfString(
+            text.FromTo(*nextPos, pos), aFontMetrics, drawTarget);
+        if (totalWidth + charWidth > aWidth) {
+          break;
+        }
+
+        pos = *nextPos;
+        totalWidth += charWidth;
+      }
+
+      if (pos > 0) {
+        aText.Replace(0, pos, kEllipsis);
+      }
+    } break;
+
+    case CropCenter: {
+      const Span text(aText);
+      nscoord totalWidth = 0;
+      GraphemeClusterBreakIteratorUtf16 leftIter(text);
+      GraphemeClusterBreakReverseIteratorUtf16 rightIter(text);
+      uint32_t leftPos = 0;
+      uint32_t rightPos = text.Length();
+
+      while (leftPos < rightPos) {
+        Maybe<uint32_t> nextPos = leftIter.Next();
+        nscoord charWidth = nsLayoutUtils::AppUnitWidthOfString(
+            text.FromTo(leftPos, *nextPos), aFontMetrics, drawTarget);
+        if (totalWidth + charWidth > aWidth) {
+          break;
+        }
+
+        leftPos = *nextPos;
+        totalWidth += charWidth;
+
+        if (leftPos >= rightPos) {
+          break;
+        }
+
+        nextPos = rightIter.Next();
+        charWidth = nsLayoutUtils::AppUnitWidthOfString(
+            text.FromTo(*nextPos, rightPos), aFontMetrics, drawTarget);
+        if (totalWidth + charWidth > aWidth) {
+          break;
+        }
+
+        rightPos = *nextPos;
+        totalWidth += charWidth;
+      }
+
+      if (leftPos < rightPos) {
+        aText.Replace(leftPos, rightPos - leftPos, kEllipsis);
+      }
+    } break;
+  }
+}
+
+// Function that cancels all the image requests in our cache.
+void nsTreeBodyFrame::CancelImageRequests() {
+  for (nsTreeImageCacheEntry entry : mImageCache.Values()) {
+    // If our imgIRequest object was registered with the refresh driver
+    // then we need to deregister it.
+    nsLayoutUtils::DeregisterImageRequest(PresContext(), entry.request,
+                                          nullptr);
+    entry.request->UnlockImage();
+    entry.request->CancelAndForgetObserver(NS_BINDING_ABORTED);
+  }
+}
+
+//
+// NS_NewTreeFrame
+//
+// Creates a new tree frame
+//
+nsIFrame* NS_NewTreeBodyFrame(PresShell* aPresShell, ComputedStyle* aStyle) {
+  return new (aPresShell) nsTreeBodyFrame(aStyle, aPresShell->GetPresContext());
+}
+
+NS_IMPL_FRAMEARENA_HELPERS(nsTreeBodyFrame)
+
+NS_QUERYFRAME_HEAD(nsTreeBodyFrame)
+  NS_QUERYFRAME_ENTRY(nsIScrollbarMediator)
+  NS_QUERYFRAME_ENTRY(nsTreeBodyFrame)
+NS_QUERYFRAME_TAIL_INHERITING(SimpleXULLeafFrame)
+
+// Constructor
+nsTreeBodyFrame::nsTreeBodyFrame(ComputedStyle* aStyle,
+                                 nsPresContext* aPresContext)
+    : SimpleXULLeafFrame(aStyle, aPresContext, kClassID),
+      mImageCache(),
+      mTopRowIndex(0),
+      mPageLength(0),
+      mHorzPosition(0),
+      mOriginalHorzWidth(-1),
+      mHorzWidth(0),
+      mAdjustWidth(0),
+      mRowHeight(0),
+      mIndentation(0),
+      mUpdateBatchNest(0),
+      mRowCount(0),
+      mMouseOverRow(-1),
+      mFocused(false),
+      mHasFixedRowCount(false),
+      mVerticalOverflow(false),
+      mHorizontalOverflow(false),
+      mReflowCallbackPosted(false),
+      mCheckingOverflow(false) {
+  mColumns = new nsTreeColumns(this);
+}
+
+// Destructor
+nsTreeBodyFrame::~nsTreeBodyFrame() {
+  CancelImageRequests();
+  DetachImageListeners();
+}
+
+static void GetBorderPadding(ComputedStyle* aStyle, nsMargin& aMargin) {
+  aMargin.SizeTo(0, 0, 0, 0);
+  aStyle->StylePadding()->GetPadding(aMargin);
+  aMargin += aStyle->StyleBorder()->GetComputedBorder();
+}
+
+static void AdjustForBorderPadding(ComputedStyle* aStyle, nsRect& aRect) {
+  nsMargin borderPadding(0, 0, 0, 0);
+  GetBorderPadding(aStyle, borderPadding);
+  aRect.Deflate(borderPadding);
+}
+
+void nsTreeBodyFrame::Init(nsIContent* aContent, nsContainerFrame* aParent,
+                           nsIFrame* aPrevInFlow) {
+  SimpleXULLeafFrame::Init(aContent, aParent, aPrevInFlow);
+
+  mIndentation = GetIndentation();
+  mRowHeight = GetRowHeight();
+
+  // Call GetBaseElement so that mTree is assigned.
+  RefPtr<XULTreeElement> tree(GetBaseElement());
+  if (MOZ_LIKELY(tree)) {
+    nsAutoString rows;
+    if (tree->GetAttr(nsGkAtoms::rows, rows)) {
+      nsresult err;
+      mPageLength = rows.ToInteger(&err);
+      mHasFixedRowCount = true;
+    }
+  }
+
+  if (PresContext()->UseOverlayScrollbars()) {
+    mScrollbarActivity =
+        new ScrollbarActivity(static_cast<nsIScrollbarMediator*>(this));
+  }
+}
+
+void nsTreeBodyFrame::Destroy(DestroyContext& aContext) {
+  if (mScrollbarActivity) {
+    mScrollbarActivity->Destroy();
+    mScrollbarActivity = nullptr;
+  }
+
+  mScrollEvent.Revoke();
+  // Make sure we cancel any posted callbacks.
+  if (mReflowCallbackPosted) {
+    PresShell()->CancelReflowCallback(this);
+    mReflowCallbackPosted = false;
+  }
+
+  if (mColumns) mColumns->SetTree(nullptr);
+
+  RefPtr tree = mTree;
+
+  if (nsCOMPtr<nsITreeView> view = std::move(mView)) {
+    nsCOMPtr<nsITreeSelection> sel;
+    view->GetSelection(getter_AddRefs(sel));
+    if (sel) {
+      sel->SetTree(nullptr);
+    }
+    view->SetTree(nullptr);
+  }
+
+  // Make this call now because view->SetTree can run js which can undo this
+  // call.
+  if (tree) {
+    tree->BodyDestroyed(mTopRowIndex);
+  }
+  if (mTree && mTree != tree) {
+    mTree->BodyDestroyed(mTopRowIndex);
+  }
+
+  SimpleXULLeafFrame::Destroy(aContext);
+}
+
+void nsTreeBodyFrame::EnsureView() {
+  if (mView) {
+    return;
+  }
+
+  if (PresShell()->IsReflowLocked()) {
+    if (!mReflowCallbackPosted) {
+      mReflowCallbackPosted = true;
+      PresShell()->PostReflowCallback(this);
+    }
+    return;
+  }
+
+  AutoWeakFrame weakFrame(this);
+
+  RefPtr<XULTreeElement> tree = GetBaseElement();
+  if (!tree) {
+    return;
+  }
+  nsCOMPtr<nsITreeView> treeView = tree->GetView();
+  if (!treeView || !weakFrame.IsAlive()) {
+    return;
+  }
+  int32_t rowIndex = tree->GetCachedTopVisibleRow();
+
+  // Set our view.
+  SetView(treeView);
+  NS_ENSURE_TRUE_VOID(weakFrame.IsAlive());
+
+  // Scroll to the given row.
+  // XXX is this optimal if we haven't laid out yet?
+  ScrollToRow(rowIndex);
+  NS_ENSURE_TRUE_VOID(weakFrame.IsAlive());
+}
+
+void nsTreeBodyFrame::ManageReflowCallback() {
+  const nscoord horzWidth = CalcHorzWidth(GetScrollParts());
+  if (!mReflowCallbackPosted) {
+    if (!mLastReflowRect || !mLastReflowRect->IsEqualEdges(mRect) ||
+        mHorzWidth != horzWidth) {
+      PresShell()->PostReflowCallback(this);
+      mReflowCallbackPosted = true;
+      mOriginalHorzWidth = mHorzWidth;
+    }
+  } else if (mHorzWidth != horzWidth && mOriginalHorzWidth == horzWidth) {
+    // FIXME(emilio): This doesn't seem sound to me, if the rect changes in the
+    // block axis.
+    PresShell()->CancelReflowCallback(this);
+    mReflowCallbackPosted = false;
+    mOriginalHorzWidth = -1;
+  }
+  mLastReflowRect = Some(mRect);
+  mHorzWidth = horzWidth;
+}
+
+nscoord nsTreeBodyFrame::GetIntrinsicBSize() {
+  return mHasFixedRowCount ? mRowHeight * mPageLength : 0;
+}
+
+void nsTreeBodyFrame::DidReflow(nsPresContext* aPresContext,
+                                const ReflowInput* aReflowInput) {
+  ManageReflowCallback();
+  SimpleXULLeafFrame::DidReflow(aPresContext, aReflowInput);
+}
+
+bool nsTreeBodyFrame::ReflowFinished() {
+  if (!mView) {
+    AutoWeakFrame weakFrame(this);
+    EnsureView();
+    NS_ENSURE_TRUE(weakFrame.IsAlive(), false);
+  }
+  if (mView) {
+    CalcInnerBox();
+    ScrollParts parts = GetScrollParts();
+    mHorzWidth = CalcHorzWidth(parts);
+    if (!mHasFixedRowCount) {
+      mPageLength =
+          (mRowHeight > 0) ? (mInnerBox.height / mRowHeight) : mRowCount;
+    }
+
+    int32_t lastPageTopRow = std::max(0, mRowCount - mPageLength);
+    if (mTopRowIndex > lastPageTopRow)
+      ScrollToRowInternal(parts, lastPageTopRow);
+
+    XULTreeElement* treeContent = GetBaseElement();
+    if (treeContent && treeContent->AttrValueIs(
+                           kNameSpaceID_None, nsGkAtoms::keepcurrentinview,
+                           nsGkAtoms::_true, eCaseMatters)) {
+      // make sure that the current selected item is still
+      // visible after the tree changes size.
+      if (nsCOMPtr<nsITreeSelection> sel = GetSelection()) {
+        int32_t currentIndex;
+        sel->GetCurrentIndex(&currentIndex);
+        if (currentIndex != -1) {
+          EnsureRowIsVisibleInternal(parts, currentIndex);
+        }
+      }
+    }
+
+    if (!FullScrollbarsUpdate(false)) {
+      return false;
+    }
+  }
+
+  mReflowCallbackPosted = false;
+  return false;
+}
+
+void nsTreeBodyFrame::ReflowCallbackCanceled() {
+  mReflowCallbackPosted = false;
+}
+
+nsresult nsTreeBodyFrame::GetView(nsITreeView** aView) {
+  *aView = nullptr;
+  AutoWeakFrame weakFrame(this);
+  EnsureView();
+  NS_ENSURE_STATE(weakFrame.IsAlive());
+  NS_IF_ADDREF(*aView = mView);
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::SetView(nsITreeView* aView) {
+  if (aView == mView) {
+    return NS_OK;
+  }
+
+  // First clear out the old view.
+  nsCOMPtr<nsITreeView> oldView = std::move(mView);
+  if (oldView) {
+    AutoWeakFrame weakFrame(this);
+
+    nsCOMPtr<nsITreeSelection> sel;
+    oldView->GetSelection(getter_AddRefs(sel));
+    if (sel) {
+      sel->SetTree(nullptr);
+    }
+    oldView->SetTree(nullptr);
+
+    NS_ENSURE_STATE(weakFrame.IsAlive());
+
+    // Only reset the top row index and delete the columns if we had an old
+    // non-null view.
+    mTopRowIndex = 0;
+  }
+
+  // Tree, meet the view.
+  mView = aView;
+
+  // Changing the view causes us to refetch our data.  This will
+  // necessarily entail a full invalidation of the tree.
+  Invalidate();
+
+  RefPtr<XULTreeElement> treeContent = GetBaseElement();
+  if (treeContent) {
+#ifdef ACCESSIBILITY
+    if (nsAccessibilityService* accService = GetAccService()) {
+      accService->TreeViewChanged(PresContext()->GetPresShell(), treeContent,
+                                  mView);
+    }
+#endif  // #ifdef ACCESSIBILITY
+    FireDOMEvent(u"TreeViewChanged"_ns, treeContent);
+  }
+
+  if (aView) {
+    // Give the view a new empty selection object to play with, but only if it
+    // doesn't have one already.
+    nsCOMPtr<nsITreeSelection> sel;
+    aView->GetSelection(getter_AddRefs(sel));
+    if (sel) {
+      sel->SetTree(treeContent);
+    } else {
+      NS_NewTreeSelection(treeContent, getter_AddRefs(sel));
+      aView->SetSelection(sel);
+    }
+
+    // View, meet the tree.
+    AutoWeakFrame weakFrame(this);
+    aView->SetTree(treeContent);
+    NS_ENSURE_STATE(weakFrame.IsAlive());
+    aView->GetRowCount(&mRowCount);
+
+    if (!PresShell()->IsReflowLocked()) {
+      // The scrollbar will need to be updated.
+      FullScrollbarsUpdate(false);
+    } else if (!mReflowCallbackPosted) {
+      mReflowCallbackPosted = true;
+      PresShell()->PostReflowCallback(this);
+    }
+  }
+
+  return NS_OK;
+}
+
+already_AddRefed<nsITreeSelection> nsTreeBodyFrame::GetSelection() const {
+  nsCOMPtr<nsITreeSelection> sel;
+  if (nsCOMPtr<nsITreeView> view = GetExistingView()) {
+    view->GetSelection(getter_AddRefs(sel));
+  }
+  return sel.forget();
+}
+
+nsresult nsTreeBodyFrame::SetFocused(bool aFocused) {
+  if (mFocused != aFocused) {
+    mFocused = aFocused;
+    if (nsCOMPtr<nsITreeSelection> sel = GetSelection()) {
+      sel->InvalidateSelection();
+    }
+  }
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::GetTreeBody(Element** aElement) {
+  // NS_ASSERTION(mContent, "no content, see bug #104878");
+  if (!mContent) return NS_ERROR_NULL_POINTER;
+
+  RefPtr<Element> element = mContent->AsElement();
+  element.forget(aElement);
+  return NS_OK;
+}
+
+int32_t nsTreeBodyFrame::RowHeight() const {
+  return nsPresContext::AppUnitsToIntCSSPixels(mRowHeight);
+}
+
+int32_t nsTreeBodyFrame::RowWidth() {
+  return nsPresContext::AppUnitsToIntCSSPixels(CalcHorzWidth(GetScrollParts()));
+}
+
+int32_t nsTreeBodyFrame::GetHorizontalPosition() const {
+  return nsPresContext::AppUnitsToIntCSSPixels(mHorzPosition);
+}
+
+Maybe<CSSIntRegion> nsTreeBodyFrame::GetSelectionRegion() {
+  if (!mView) {
+    return Nothing();
+  }
+
+  AutoWeakFrame wf(this);
+  nsCOMPtr<nsITreeSelection> selection = GetSelection();
+  if (!selection || !wf.IsAlive()) {
+    return Nothing();
+  }
+
+  RefPtr<nsPresContext> presContext = PresContext();
+  nsIntRect rect = mRect.ToOutsidePixels(AppUnitsPerCSSPixel());
+
+  nsIFrame* rootFrame = presContext->PresShell()->GetRootFrame();
+  nsPoint origin = GetOffsetTo(rootFrame);
+
+  CSSIntRegion region;
+
+  // iterate through the visible rows and add the selected ones to the
+  // drag region
+  int32_t x = nsPresContext::AppUnitsToIntCSSPixels(origin.x);
+  int32_t y = nsPresContext::AppUnitsToIntCSSPixels(origin.y);
+  int32_t top = y;
+  int32_t end = LastVisibleRow();
+  int32_t rowHeight = nsPresContext::AppUnitsToIntCSSPixels(mRowHeight);
+  for (int32_t i = mTopRowIndex; i <= end; i++) {
+    bool isSelected;
+    selection->IsSelected(i, &isSelected);
+    if (isSelected) {
+      region.OrWith(CSSIntRect(x, y, rect.width, rowHeight));
+    }
+    y += rowHeight;
+  }
+
+  // clip to the tree boundary in case one row extends past it
+  region.AndWith(CSSIntRect(x, top, rect.width, rect.height));
+
+  return Some(region);
+}
+
+nsresult nsTreeBodyFrame::Invalidate() {
+  if (mUpdateBatchNest) return NS_OK;
+
+  InvalidateFrame();
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::InvalidateColumn(nsTreeColumn* aCol) {
+  if (mUpdateBatchNest) return NS_OK;
+
+  if (!aCol) return NS_ERROR_INVALID_ARG;
+
+#ifdef ACCESSIBILITY
+  if (GetAccService()) {
+    FireInvalidateEvent(-1, -1, aCol, aCol);
+  }
+#endif  // #ifdef ACCESSIBILITY
+
+  nsRect columnRect;
+  nsresult rv = aCol->GetRect(this, mInnerBox.y, mInnerBox.height, &columnRect);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // When false then column is out of view
+  if (OffsetForHorzScroll(columnRect, true))
+    InvalidateFrameWithRect(columnRect);
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::InvalidateRow(int32_t aIndex) {
+  if (mUpdateBatchNest) return NS_OK;
+
+#ifdef ACCESSIBILITY
+  if (GetAccService()) {
+    FireInvalidateEvent(aIndex, aIndex, nullptr, nullptr);
+  }
+#endif  // #ifdef ACCESSIBILITY
+
+  aIndex -= mTopRowIndex;
+  if (aIndex < 0 || aIndex > mPageLength) return NS_OK;
+
+  nsRect rowRect(mInnerBox.x, mInnerBox.y + mRowHeight * aIndex,
+                 mInnerBox.width, mRowHeight);
+  InvalidateFrameWithRect(rowRect);
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::InvalidateCell(int32_t aIndex, nsTreeColumn* aCol) {
+  if (mUpdateBatchNest) return NS_OK;
+
+#ifdef ACCESSIBILITY
+  if (GetAccService()) {
+    FireInvalidateEvent(aIndex, aIndex, aCol, aCol);
+  }
+#endif  // #ifdef ACCESSIBILITY
+
+  aIndex -= mTopRowIndex;
+  if (aIndex < 0 || aIndex > mPageLength) return NS_OK;
+
+  if (!aCol) return NS_ERROR_INVALID_ARG;
+
+  nsRect cellRect;
+  nsresult rv = aCol->GetRect(this, mInnerBox.y + mRowHeight * aIndex,
+                              mRowHeight, &cellRect);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (OffsetForHorzScroll(cellRect, true)) InvalidateFrameWithRect(cellRect);
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::InvalidateRange(int32_t aStart, int32_t aEnd) {
+  if (mUpdateBatchNest) return NS_OK;
+
+  if (aStart == aEnd) return InvalidateRow(aStart);
+
+  int32_t last = LastVisibleRow();
+  if (aStart > aEnd || aEnd < mTopRowIndex || aStart > last) return NS_OK;
+
+  if (aStart < mTopRowIndex) aStart = mTopRowIndex;
+
+  if (aEnd > last) aEnd = last;
+
+#ifdef ACCESSIBILITY
+  if (GetAccService()) {
+    int32_t end =
+        mRowCount > 0 ? ((mRowCount <= aEnd) ? mRowCount - 1 : aEnd) : 0;
+    FireInvalidateEvent(aStart, end, nullptr, nullptr);
+  }
+#endif  // #ifdef ACCESSIBILITY
+
+  nsRect rangeRect(mInnerBox.x,
+                   mInnerBox.y + mRowHeight * (aStart - mTopRowIndex),
+                   mInnerBox.width, mRowHeight * (aEnd - aStart + 1));
+  InvalidateFrameWithRect(rangeRect);
+
+  return NS_OK;
+}
+
+static void FindScrollParts(nsIFrame* aCurrFrame,
+                            nsTreeBodyFrame::ScrollParts* aResult) {
+  if (!aResult->mColumnsScrollFrame) {
+    nsIScrollableFrame* f = do_QueryFrame(aCurrFrame);
+    if (f) {
+      aResult->mColumnsFrame = aCurrFrame;
+      aResult->mColumnsScrollFrame = f;
+    }
+  }
+
+  if (nsScrollbarFrame* sf = do_QueryFrame(aCurrFrame)) {
+    if (!sf->IsHorizontal()) {
+      if (!aResult->mVScrollbar) {
+        aResult->mVScrollbar = sf;
+      }
+    } else {
+      if (!aResult->mHScrollbar) {
+        aResult->mHScrollbar = sf;
+      }
+    }
+    // don't bother searching inside a scrollbar
+    return;
+  }
+
+  nsIFrame* child = aCurrFrame->PrincipalChildList().FirstChild();
+  while (child && !child->GetContent()->IsRootOfNativeAnonymousSubtree() &&
+         (!aResult->mVScrollbar || !aResult->mHScrollbar ||
+          !aResult->mColumnsScrollFrame)) {
+    FindScrollParts(child, aResult);
+    child = child->GetNextSibling();
+  }
+}
+
+nsTreeBodyFrame::ScrollParts nsTreeBodyFrame::GetScrollParts() {
+  ScrollParts result = {nullptr, nullptr, nullptr, nullptr, nullptr, nullptr};
+  XULTreeElement* tree = GetBaseElement();
+  if (nsIFrame* treeFrame = tree ? tree->GetPrimaryFrame() : nullptr) {
+    // The way we do this, searching through the entire frame subtree, is pretty
+    // dumb! We should know where these frames are.
+    FindScrollParts(treeFrame, &result);
+    if (result.mHScrollbar) {
+      result.mHScrollbar->SetScrollbarMediatorContent(GetContent());
+      result.mHScrollbarContent = result.mHScrollbar->GetContent()->AsElement();
+    }
+    if (result.mVScrollbar) {
+      result.mVScrollbar->SetScrollbarMediatorContent(GetContent());
+      result.mVScrollbarContent = result.mVScrollbar->GetContent()->AsElement();
+    }
+  }
+  return result;
+}
+
+void nsTreeBodyFrame::UpdateScrollbars(const ScrollParts& aParts) {
+  nscoord rowHeightAsPixels = nsPresContext::AppUnitsToIntCSSPixels(mRowHeight);
+
+  AutoWeakFrame weakFrame(this);
+
+  if (aParts.mVScrollbar) {
+    nsAutoString curPos;
+    curPos.AppendInt(mTopRowIndex * rowHeightAsPixels);
+    aParts.mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::curpos,
+                                       curPos, true);
+    // 'this' might be deleted here
+  }
+
+  if (weakFrame.IsAlive() && aParts.mHScrollbar) {
+    nsAutoString curPos;
+    curPos.AppendInt(mHorzPosition);
+    aParts.mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::curpos,
+                                       curPos, true);
+    // 'this' might be deleted here
+  }
+
+  if (weakFrame.IsAlive() && mScrollbarActivity) {
+    mScrollbarActivity->ActivityOccurred();
+  }
+}
+
+void nsTreeBodyFrame::CheckOverflow(const ScrollParts& aParts) {
+  bool verticalOverflowChanged = false;
+  bool horizontalOverflowChanged = false;
+
+  if (!mVerticalOverflow && mRowCount > mPageLength) {
+    mVerticalOverflow = true;
+    verticalOverflowChanged = true;
+  } else if (mVerticalOverflow && mRowCount <= mPageLength) {
+    mVerticalOverflow = false;
+    verticalOverflowChanged = true;
+  }
+
+  if (aParts.mColumnsFrame) {
+    nsRect bounds = aParts.mColumnsFrame->GetRect();
+    if (bounds.width != 0) {
+      /* Ignore overflows that are less than half a pixel. Yes these happen
+         all over the place when flex boxes are compressed real small.
+         Probably a result of a rounding errors somewhere in the layout code. */
+      bounds.width += nsPresContext::CSSPixelsToAppUnits(0.5f);
+      if (!mHorizontalOverflow && bounds.width < mHorzWidth) {
+        mHorizontalOverflow = true;
+        horizontalOverflowChanged = true;
+      } else if (mHorizontalOverflow && bounds.width >= mHorzWidth) {
+        mHorizontalOverflow = false;
+        horizontalOverflowChanged = true;
+      }
+    }
+  }
+
+  if (!horizontalOverflowChanged && !verticalOverflowChanged) {
+    return;
+  }
+
+  AutoWeakFrame weakFrame(this);
+
+  RefPtr<nsPresContext> presContext = PresContext();
+  RefPtr<mozilla::PresShell> presShell = presContext->GetPresShell();
+  nsCOMPtr<nsIContent> content = mContent;
+
+  if (verticalOverflowChanged) {
+    InternalScrollPortEvent event(
+        true, mVerticalOverflow ? eScrollPortOverflow : eScrollPortUnderflow,
+        nullptr);
+    event.mOrient = InternalScrollPortEvent::eVertical;
+    EventDispatcher::Dispatch(content, presContext, &event);
+  }
+
+  if (horizontalOverflowChanged) {
+    InternalScrollPortEvent event(
+        true, mHorizontalOverflow ? eScrollPortOverflow : eScrollPortUnderflow,
+        nullptr);
+    event.mOrient = InternalScrollPortEvent::eHorizontal;
+    EventDispatcher::Dispatch(content, presContext, &event);
+  }
+
+  // The synchronous event dispatch above can trigger reflow notifications.
+  // Flush those explicitly now, so that we can guard against potential infinite
+  // recursion. See bug 905909.
+  if (!weakFrame.IsAlive()) {
+    return;
+  }
+  NS_ASSERTION(!mCheckingOverflow,
+               "mCheckingOverflow should not already be set");
+  // Don't use AutoRestore since we want to not touch mCheckingOverflow if we
+  // fail the weakFrame.IsAlive() check below
+  mCheckingOverflow = true;
+  presShell->FlushPendingNotifications(FlushType::Layout);
+  if (!weakFrame.IsAlive()) {
+    return;
+  }
+  mCheckingOverflow = false;
+}
+
+void nsTreeBodyFrame::InvalidateScrollbars(const ScrollParts& aParts,
+                                           AutoWeakFrame& aWeakColumnsFrame) {
+  if (mUpdateBatchNest || !mView) return;
+  AutoWeakFrame weakFrame(this);
+
+  if (aParts.mVScrollbar) {
+    // Do Vertical Scrollbar
+    nsAutoString maxposStr;
+
+    nscoord rowHeightAsPixels =
+        nsPresContext::AppUnitsToIntCSSPixels(mRowHeight);
+
+    int32_t size = rowHeightAsPixels *
+                   (mRowCount > mPageLength ? mRowCount - mPageLength : 0);
+    maxposStr.AppendInt(size);
+    aParts.mVScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::maxpos,
+                                       maxposStr, true);
+    NS_ENSURE_TRUE_VOID(weakFrame.IsAlive());
+
+    // Also set our page increment and decrement.
+    nscoord pageincrement = mPageLength * rowHeightAsPixels;
+    nsAutoString pageStr;
+    pageStr.AppendInt(pageincrement);
+    aParts.mVScrollbarContent->SetAttr(kNameSpaceID_None,
+                                       nsGkAtoms::pageincrement, pageStr, true);
+    NS_ENSURE_TRUE_VOID(weakFrame.IsAlive());
+  }
+
+  if (aParts.mHScrollbar && aParts.mColumnsFrame &&
+      aWeakColumnsFrame.IsAlive()) {
+    // And now Horizontal scrollbar
+    nsRect bounds = aParts.mColumnsFrame->GetRect();
+    nsAutoString maxposStr;
+
+    maxposStr.AppendInt(mHorzWidth > bounds.width ? mHorzWidth - bounds.width
+                                                  : 0);
+    aParts.mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::maxpos,
+                                       maxposStr, true);
+    NS_ENSURE_TRUE_VOID(weakFrame.IsAlive());
+
+    nsAutoString pageStr;
+    pageStr.AppendInt(bounds.width);
+    aParts.mHScrollbarContent->SetAttr(kNameSpaceID_None,
+                                       nsGkAtoms::pageincrement, pageStr, true);
+    NS_ENSURE_TRUE_VOID(weakFrame.IsAlive());
+
+    pageStr.Truncate();
+    pageStr.AppendInt(nsPresContext::CSSPixelsToAppUnits(16));
+    aParts.mHScrollbarContent->SetAttr(kNameSpaceID_None, nsGkAtoms::increment,
+                                       pageStr, true);
+  }
+
+  if (weakFrame.IsAlive() && mScrollbarActivity) {
+    mScrollbarActivity->ActivityOccurred();
+  }
+}
+
+// Takes client x/y in pixels, converts them to appunits, and converts into
+// values relative to this nsTreeBodyFrame frame.
+nsPoint nsTreeBodyFrame::AdjustClientCoordsToBoxCoordSpace(int32_t aX,
+                                                           int32_t aY) {
+  nsPoint point(nsPresContext::CSSPixelsToAppUnits(aX),
+                nsPresContext::CSSPixelsToAppUnits(aY));
+
+  nsPresContext* presContext = PresContext();
+  point -= GetOffsetTo(presContext->GetPresShell()->GetRootFrame());
+
+  // Adjust by the inner box coords, so that we're in the inner box's
+  // coordinate space.
+  point -= mInnerBox.TopLeft();
+  return point;
+}  // AdjustClientCoordsToBoxCoordSpace
+
+int32_t nsTreeBodyFrame::GetRowAt(int32_t aX, int32_t aY) {
+  if (!mView) {
+    return 0;
+  }
+
+  nsPoint point = AdjustClientCoordsToBoxCoordSpace(aX, aY);
+
+  // Check if the coordinates are above our visible space.
+  if (point.y < 0) {
+    return -1;
+  }
+
+  return GetRowAtInternal(point.x, point.y);
+}
+
+nsresult nsTreeBodyFrame::GetCellAt(int32_t aX, int32_t aY, int32_t* aRow,
+                                    nsTreeColumn** aCol,
+                                    nsACString& aChildElt) {
+  if (!mView) return NS_OK;
+
+  nsPoint point = AdjustClientCoordsToBoxCoordSpace(aX, aY);
+
+  // Check if the coordinates are above our visible space.
+  if (point.y < 0) {
+    *aRow = -1;
+    return NS_OK;
+  }
+
+  nsTreeColumn* col;
+  nsCSSAnonBoxPseudoStaticAtom* child;
+  GetCellAt(point.x, point.y, aRow, &col, &child);
+
+  if (col) {
+    NS_ADDREF(*aCol = col);
+    if (child == nsCSSAnonBoxes::mozTreeCell())
+      aChildElt.AssignLiteral("cell");
+    else if (child == nsCSSAnonBoxes::mozTreeTwisty())
+      aChildElt.AssignLiteral("twisty");
+    else if (child == nsCSSAnonBoxes::mozTreeImage())
+      aChildElt.AssignLiteral("image");
+    else if (child == nsCSSAnonBoxes::mozTreeCellText())
+      aChildElt.AssignLiteral("text");
+  }
+
+  return NS_OK;
+}
+
+//
+// GetCoordsForCellItem
+//
+// Find the x/y location and width/height (all in PIXELS) of the given object
+// in the given column.
+//
+// XXX IMPORTANT XXX:
+// Hyatt says in the bug for this, that the following needs to be done:
+// (1) You need to deal with overflow when computing cell rects.  See other
+// column iteration examples... if you don't deal with this, you'll mistakenly
+// extend the cell into the scrollbar's rect.
+//
+// (2) You are adjusting the cell rect by the *row" border padding.  That's
+// wrong.  You need to first adjust a row rect by its border/padding, and then
+// the cell rect fits inside the adjusted row rect.  It also can have
+// border/padding as well as margins.  The vertical direction isn't that
+// important, but you need to get the horizontal direction right.
+//
+// (3) GetImageSize() does not include margins (but it does include
+// border/padding). You need to make sure to add in the image's margins as well.
+//
+nsresult nsTreeBodyFrame::GetCoordsForCellItem(int32_t aRow, nsTreeColumn* aCol,
+                                               const nsACString& aElement,
+                                               int32_t* aX, int32_t* aY,
+                                               int32_t* aWidth,
+                                               int32_t* aHeight) {
+  *aX = 0;
+  *aY = 0;
+  *aWidth = 0;
+  *aHeight = 0;
+
+  bool isRTL = StyleVisibility()->mDirection == StyleDirection::Rtl;
+  nscoord currX = mInnerBox.x - mHorzPosition;
+
+  // The Rect for the requested item.
+  nsRect theRect;
+
+  nsPresContext* presContext = PresContext();
+
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+
+  for (nsTreeColumn* currCol = mColumns->GetFirstColumn(); currCol;
+       currCol = currCol->GetNext()) {
+    // The Rect for the current cell.
+    nscoord colWidth;
+#ifdef DEBUG
+    nsresult rv =
+#endif
+        currCol->GetWidthInTwips(this, &colWidth);
+    NS_ASSERTION(NS_SUCCEEDED(rv), "invalid column");
+
+    nsRect cellRect(currX, mInnerBox.y + mRowHeight * (aRow - mTopRowIndex),
+                    colWidth, mRowHeight);
+
+    // Check the ID of the current column to see if it matches. If it doesn't
+    // increment the current X value and continue to the next column.
+    if (currCol != aCol) {
+      currX += cellRect.width;
+      continue;
+    }
+    // Now obtain the properties for our cell.
+    PrefillPropertyArray(aRow, currCol);
+
+    nsAutoString properties;
+    view->GetCellProperties(aRow, currCol, properties);
+    nsTreeUtils::TokenizeProperties(properties, mScratchArray);
+
+    ComputedStyle* rowContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeRow());
+
+    // We don't want to consider any of the decorations that may be present
+    // on the current row, so we have to deflate the rect by the border and
+    // padding and offset its left and top coordinates appropriately.
+    AdjustForBorderPadding(rowContext, cellRect);
+
+    ComputedStyle* cellContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCell());
+
+    constexpr auto cell = "cell"_ns;
+    if (currCol->IsCycler() || cell.Equals(aElement)) {
+      // If the current Column is a Cycler, then the Rect is just the cell - the
+      // margins. Similarly, if we're just being asked for the cell rect,
+      // provide it.
+
+      theRect = cellRect;
+      nsMargin cellMargin;
+      cellContext->StyleMargin()->GetMargin(cellMargin);
+      theRect.Deflate(cellMargin);
+      break;
+    }
+
+    // Since we're not looking for the cell, and since the cell isn't a cycler,
+    // we're looking for some subcomponent, and now we need to subtract the
+    // borders and padding of the cell from cellRect so this does not
+    // interfere with our computations.
+    AdjustForBorderPadding(cellContext, cellRect);
+
+    UniquePtr<gfxContext> rc =
+        presContext->PresShell()->CreateReferenceRenderingContext();
+
+    // Now we'll start making our way across the cell, starting at the edge of
+    // the cell and proceeding until we hit the right edge. |cellX| is the
+    // working X value that we will increment as we crawl from left to right.
+    nscoord cellX = cellRect.x;
+    nscoord remainWidth = cellRect.width;
+
+    if (currCol->IsPrimary()) {
+      // If the current Column is a Primary, then we need to take into account
+      // the indentation and possibly a twisty.
+
+      // The amount of indentation is the indentation width (|mIndentation|) by
+      // the level.
+      int32_t level;
+      view->GetLevel(aRow, &level);
+      if (!isRTL) cellX += mIndentation * level;
+      remainWidth -= mIndentation * level;
+
+      // Find the twisty rect by computing its size.
+      nsRect imageRect;
+      nsRect twistyRect(cellRect);
+      ComputedStyle* twistyContext =
+          GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeTwisty());
+      GetTwistyRect(aRow, currCol, imageRect, twistyRect, presContext,
+                    twistyContext);
+
+      if ("twisty"_ns.Equals(aElement)) {
+        // If we're looking for the twisty Rect, just return the size
+        theRect = twistyRect;
+        break;
+      }
+
+      // Now we need to add in the margins of the twisty element, so that we
+      // can find the offset of the next element in the cell.
+      nsMargin twistyMargin;
+      twistyContext->StyleMargin()->GetMargin(twistyMargin);
+      twistyRect.Inflate(twistyMargin);
+
+      // Adjust our working X value with the twisty width (image size, margins,
+      // borders, padding.
+      if (!isRTL) cellX += twistyRect.width;
+    }
+
+    // Cell Image
+    ComputedStyle* imageContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeImage());
+
+    nsRect imageSize = GetImageSize(aRow, currCol, false, imageContext);
+    if ("image"_ns.Equals(aElement)) {
+      theRect = imageSize;
+      theRect.x = cellX;
+      theRect.y = cellRect.y;
+      break;
+    }
+
+    // Add in the margins of the cell image.
+    nsMargin imageMargin;
+    imageContext->StyleMargin()->GetMargin(imageMargin);
+    imageSize.Inflate(imageMargin);
+
+    // Increment cellX by the image width
+    if (!isRTL) cellX += imageSize.width;
+
+    // Cell Text
+    nsAutoString cellText;
+    view->GetCellText(aRow, currCol, cellText);
+    // We're going to measure this text so we need to ensure bidi is enabled if
+    // necessary
+    CheckTextForBidi(cellText);
+
+    // Create a scratch rect to represent the text rectangle, with the current
+    // X and Y coords, and a guess at the width and height. The width is the
+    // remaining width we have left to traverse in the cell, which will be the
+    // widest possible value for the text rect, and the row height.
+    nsRect textRect(cellX, cellRect.y, remainWidth, cellRect.height);
+
+    // Measure the width of the text. If the width of the text is greater than
+    // the remaining width available, then we just assume that the text has
+    // been cropped and use the remaining rect as the text Rect. Otherwise,
+    // we add in borders and padding to the text dimension and give that back.
+    ComputedStyle* textContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCellText());
+
+    RefPtr<nsFontMetrics> fm =
+        nsLayoutUtils::GetFontMetricsForComputedStyle(textContext, presContext);
+    nscoord height = fm->MaxHeight();
+
+    nsMargin textMargin;
+    textContext->StyleMargin()->GetMargin(textMargin);
+    textRect.Deflate(textMargin);
+
+    // Center the text. XXX Obey vertical-align style prop?
+    if (height < textRect.height) {
+      textRect.y += (textRect.height - height) / 2;
+      textRect.height = height;
+    }
+
+    nsMargin bp(0, 0, 0, 0);
+    GetBorderPadding(textContext, bp);
+    textRect.height += bp.top + bp.bottom;
+
+    AdjustForCellText(cellText, aRow, currCol, *rc, *fm, textRect);
+
+    theRect = textRect;
+  }
+
+  if (isRTL) theRect.x = mInnerBox.width - theRect.x - theRect.width;
+
+  *aX = nsPresContext::AppUnitsToIntCSSPixels(theRect.x);
+  *aY = nsPresContext::AppUnitsToIntCSSPixels(theRect.y);
+  *aWidth = nsPresContext::AppUnitsToIntCSSPixels(theRect.width);
+  *aHeight = nsPresContext::AppUnitsToIntCSSPixels(theRect.height);
+
+  return NS_OK;
+}
+
+int32_t nsTreeBodyFrame::GetRowAtInternal(nscoord aX, nscoord aY) {
+  if (mRowHeight <= 0) return -1;
+
+  // Now just mod by our total inner box height and add to our top row index.
+  int32_t row = (aY / mRowHeight) + mTopRowIndex;
+
+  // Check if the coordinates are below our visible space (or within our visible
+  // space but below any row).
+  if (row > mTopRowIndex + mPageLength || row >= mRowCount) return -1;
+
+  return row;
+}
+
+void nsTreeBodyFrame::CheckTextForBidi(nsAutoString& aText) {
+  // We could check to see whether the prescontext already has bidi enabled,
+  // but usually it won't, so it's probably faster to avoid the call to
+  // GetPresContext() when it's not needed.
+  if (HasRTLChars(aText)) {
+    PresContext()->SetBidiEnabled();
+  }
+}
+
+void nsTreeBodyFrame::AdjustForCellText(nsAutoString& aText, int32_t aRowIndex,
+                                        nsTreeColumn* aColumn,
+                                        gfxContext& aRenderingContext,
+                                        nsFontMetrics& aFontMetrics,
+                                        nsRect& aTextRect) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  DrawTarget* drawTarget = aRenderingContext.GetDrawTarget();
+
+  nscoord maxWidth = aTextRect.width;
+  bool widthIsGreater = nsLayoutUtils::StringWidthIsGreaterThan(
+      aText, aFontMetrics, drawTarget, maxWidth);
+
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  if (aColumn->Overflow()) {
+    DebugOnly<nsresult> rv;
+    nsTreeColumn* nextColumn = aColumn->GetNext();
+    while (nextColumn && widthIsGreater) {
+      while (nextColumn) {
+        nscoord width;
+        rv = nextColumn->GetWidthInTwips(this, &width);
+        NS_ASSERTION(NS_SUCCEEDED(rv), "nextColumn is invalid");
+
+        if (width != 0) {
+          break;
+        }
+
+        nextColumn = nextColumn->GetNext();
+      }
+
+      if (nextColumn) {
+        nsAutoString nextText;
+        view->GetCellText(aRowIndex, nextColumn, nextText);
+        // We don't measure or draw this text so no need to check it for
+        // bidi-ness
+
+        if (nextText.Length() == 0) {
+          nscoord width;
+          rv = nextColumn->GetWidthInTwips(this, &width);
+          NS_ASSERTION(NS_SUCCEEDED(rv), "nextColumn is invalid");
+
+          maxWidth += width;
+          widthIsGreater = nsLayoutUtils::StringWidthIsGreaterThan(
+              aText, aFontMetrics, drawTarget, maxWidth);
+
+          nextColumn = nextColumn->GetNext();
+        } else {
+          nextColumn = nullptr;
+        }
+      }
+    }
+  }
+
+  CroppingStyle cropType = CroppingStyle::CropRight;
+  if (aColumn->GetCropStyle() == 1) {
+    cropType = CroppingStyle::CropCenter;
+  } else if (aColumn->GetCropStyle() == 2) {
+    cropType = CroppingStyle::CropLeft;
+  }
+  CropStringForWidth(aText, aRenderingContext, aFontMetrics, maxWidth,
+                     cropType);
+
+  nscoord width = nsLayoutUtils::AppUnitWidthOfStringBidi(
+      aText, this, aFontMetrics, aRenderingContext);
+
+  switch (aColumn->GetTextAlignment()) {
+    case mozilla::StyleTextAlign::Right:
+      aTextRect.x += aTextRect.width - width;
+      break;
+    case mozilla::StyleTextAlign::Center:
+      aTextRect.x += (aTextRect.width - width) / 2;
+      break;
+    default:
+      break;
+  }
+
+  aTextRect.width = width;
+}
+
+nsCSSAnonBoxPseudoStaticAtom* nsTreeBodyFrame::GetItemWithinCellAt(
+    nscoord aX, const nsRect& aCellRect, int32_t aRowIndex,
+    nsTreeColumn* aColumn) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  // Obtain the properties for our cell.
+  PrefillPropertyArray(aRowIndex, aColumn);
+  nsAutoString properties;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  view->GetCellProperties(aRowIndex, aColumn, properties);
+  nsTreeUtils::TokenizeProperties(properties, mScratchArray);
+
+  // Resolve style for the cell.
+  ComputedStyle* cellContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCell());
+
+  // Obtain the margins for the cell and then deflate our rect by that
+  // amount.  The cell is assumed to be contained within the deflated rect.
+  nsRect cellRect(aCellRect);
+  nsMargin cellMargin;
+  cellContext->StyleMargin()->GetMargin(cellMargin);
+  cellRect.Deflate(cellMargin);
+
+  // Adjust the rect for its border and padding.
+  AdjustForBorderPadding(cellContext, cellRect);
+
+  if (aX < cellRect.x || aX >= cellRect.x + cellRect.width) {
+    // The user clicked within the cell's margins/borders/padding.  This
+    // constitutes a click on the cell.
+    return nsCSSAnonBoxes::mozTreeCell();
+  }
+
+  nscoord currX = cellRect.x;
+  nscoord remainingWidth = cellRect.width;
+
+  // Handle right alignment hit testing.
+  bool isRTL = StyleVisibility()->mDirection == StyleDirection::Rtl;
+
+  nsPresContext* presContext = PresContext();
+  UniquePtr<gfxContext> rc =
+      presContext->PresShell()->CreateReferenceRenderingContext();
+
+  if (aColumn->IsPrimary()) {
+    // If we're the primary column, we have indentation and a twisty.
+    int32_t level;
+    view->GetLevel(aRowIndex, &level);
+
+    if (!isRTL) currX += mIndentation * level;
+    remainingWidth -= mIndentation * level;
+
+    if ((isRTL && aX > currX + remainingWidth) || (!isRTL && aX < currX)) {
+      // The user clicked within the indentation.
+      return nsCSSAnonBoxes::mozTreeCell();
+    }
+
+    // Always leave space for the twisty.
+    nsRect twistyRect(currX, cellRect.y, remainingWidth, cellRect.height);
+    bool hasTwisty = false;
+    bool isContainer = false;
+    view->IsContainer(aRowIndex, &isContainer);
+    if (isContainer) {
+      bool isContainerEmpty = false;
+      view->IsContainerEmpty(aRowIndex, &isContainerEmpty);
+      if (!isContainerEmpty) hasTwisty = true;
+    }
+
+    // Resolve style for the twisty.
+    ComputedStyle* twistyContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeTwisty());
+
+    nsRect imageSize;
+    GetTwistyRect(aRowIndex, aColumn, imageSize, twistyRect, presContext,
+                  twistyContext);
+
+    // We will treat a click as hitting the twisty if it happens on the margins,
+    // borders, padding, or content of the twisty object.  By allowing a "slop"
+    // into the margin, we make it a little bit easier for a user to hit the
+    // twisty.  (We don't want to be too picky here.)
+    nsMargin twistyMargin;
+    twistyContext->StyleMargin()->GetMargin(twistyMargin);
+    twistyRect.Inflate(twistyMargin);
+    if (isRTL) twistyRect.x = currX + remainingWidth - twistyRect.width;
+
+    // Now we test to see if aX is actually within the twistyRect.  If it is,
+    // and if the item should have a twisty, then we return "twisty".  If it is
+    // within the rect but we shouldn't have a twisty, then we return "cell".
+    if (aX >= twistyRect.x && aX < twistyRect.x + twistyRect.width) {
+      if (hasTwisty)
+        return nsCSSAnonBoxes::mozTreeTwisty();
+      else
+        return nsCSSAnonBoxes::mozTreeCell();
+    }
+
+    if (!isRTL) currX += twistyRect.width;
+    remainingWidth -= twistyRect.width;
+  }
+
+  // Now test to see if the user hit the icon for the cell.
+  nsRect iconRect(currX, cellRect.y, remainingWidth, cellRect.height);
+
+  // Resolve style for the image.
+  ComputedStyle* imageContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeImage());
+
+  nsRect iconSize = GetImageSize(aRowIndex, aColumn, false, imageContext);
+  nsMargin imageMargin;
+  imageContext->StyleMargin()->GetMargin(imageMargin);
+  iconSize.Inflate(imageMargin);
+  iconRect.width = iconSize.width;
+  if (isRTL) iconRect.x = currX + remainingWidth - iconRect.width;
+
+  if (aX >= iconRect.x && aX < iconRect.x + iconRect.width) {
+    // The user clicked on the image.
+    return nsCSSAnonBoxes::mozTreeImage();
+  }
+
+  if (!isRTL) currX += iconRect.width;
+  remainingWidth -= iconRect.width;
+
+  nsAutoString cellText;
+  view->GetCellText(aRowIndex, aColumn, cellText);
+  // We're going to measure this text so we need to ensure bidi is enabled if
+  // necessary
+  CheckTextForBidi(cellText);
+
+  nsRect textRect(currX, cellRect.y, remainingWidth, cellRect.height);
+
+  ComputedStyle* textContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCellText());
+
+  nsMargin textMargin;
+  textContext->StyleMargin()->GetMargin(textMargin);
+  textRect.Deflate(textMargin);
+
+  AdjustForBorderPadding(textContext, textRect);
+
+  RefPtr<nsFontMetrics> fm =
+      nsLayoutUtils::GetFontMetricsForComputedStyle(textContext, presContext);
+  AdjustForCellText(cellText, aRowIndex, aColumn, *rc, *fm, textRect);
+
+  if (aX >= textRect.x && aX < textRect.x + textRect.width)
+    return nsCSSAnonBoxes::mozTreeCellText();
+  else
+    return nsCSSAnonBoxes::mozTreeCell();
+}
+
+void nsTreeBodyFrame::GetCellAt(nscoord aX, nscoord aY, int32_t* aRow,
+                                nsTreeColumn** aCol,
+                                nsCSSAnonBoxPseudoStaticAtom** aChildElt) {
+  *aCol = nullptr;
+  *aChildElt = nullptr;
+
+  *aRow = GetRowAtInternal(aX, aY);
+  if (*aRow < 0) return;
+
+  // Determine the column hit.
+  for (nsTreeColumn* currCol = mColumns->GetFirstColumn(); currCol;
+       currCol = currCol->GetNext()) {
+    nsRect cellRect;
+    nsresult rv = currCol->GetRect(
+        this, mInnerBox.y + mRowHeight * (*aRow - mTopRowIndex), mRowHeight,
+        &cellRect);
+    if (NS_FAILED(rv)) {
+      MOZ_ASSERT_UNREACHABLE("column has no frame");
+      continue;
+    }
+
+    if (!OffsetForHorzScroll(cellRect, false)) continue;
+
+    if (aX >= cellRect.x && aX < cellRect.x + cellRect.width) {
+      // We know the column hit now.
+      *aCol = currCol;
+
+      if (currCol->IsCycler())
+        // Cyclers contain only images.  Fill this in immediately and return.
+        *aChildElt = nsCSSAnonBoxes::mozTreeImage();
+      else
+        *aChildElt = GetItemWithinCellAt(aX, cellRect, *aRow, currCol);
+      break;
+    }
+  }
+}
+
+nsresult nsTreeBodyFrame::GetCellWidth(int32_t aRow, nsTreeColumn* aCol,
+                                       gfxContext* aRenderingContext,
+                                       nscoord& aDesiredSize,
+                                       nscoord& aCurrentSize) {
+  MOZ_ASSERT(aCol, "aCol must not be null");
+  MOZ_ASSERT(aRenderingContext, "aRenderingContext must not be null");
+
+  // The rect for the current cell.
+  nscoord colWidth;
+  nsresult rv = aCol->GetWidthInTwips(this, &colWidth);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsRect cellRect(0, 0, colWidth, mRowHeight);
+
+  int32_t overflow =
+      cellRect.x + cellRect.width - (mInnerBox.x + mInnerBox.width);
+  if (overflow > 0) cellRect.width -= overflow;
+
+  // Adjust borders and padding for the cell.
+  ComputedStyle* cellContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCell());
+  nsMargin bp(0, 0, 0, 0);
+  GetBorderPadding(cellContext, bp);
+
+  aCurrentSize = cellRect.width;
+  aDesiredSize = bp.left + bp.right;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+
+  if (aCol->IsPrimary()) {
+    // If the current Column is a Primary, then we need to take into account
+    // the indentation and possibly a twisty.
+
+    // The amount of indentation is the indentation width (|mIndentation|) by
+    // the level.
+    int32_t level;
+    view->GetLevel(aRow, &level);
+    aDesiredSize += mIndentation * level;
+
+    // Find the twisty rect by computing its size.
+    ComputedStyle* twistyContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeTwisty());
+
+    nsRect imageSize;
+    nsRect twistyRect(cellRect);
+    GetTwistyRect(aRow, aCol, imageSize, twistyRect, PresContext(),
+                  twistyContext);
+
+    // Add in the margins of the twisty element.
+    nsMargin twistyMargin;
+    twistyContext->StyleMargin()->GetMargin(twistyMargin);
+    twistyRect.Inflate(twistyMargin);
+
+    aDesiredSize += twistyRect.width;
+  }
+
+  ComputedStyle* imageContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeImage());
+
+  // Account for the width of the cell image.
+  nsRect imageSize = GetImageSize(aRow, aCol, false, imageContext);
+  // Add in the margins of the cell image.
+  nsMargin imageMargin;
+  imageContext->StyleMargin()->GetMargin(imageMargin);
+  imageSize.Inflate(imageMargin);
+
+  aDesiredSize += imageSize.width;
+
+  // Get the cell text.
+  nsAutoString cellText;
+  view->GetCellText(aRow, aCol, cellText);
+  // We're going to measure this text so we need to ensure bidi is enabled if
+  // necessary
+  CheckTextForBidi(cellText);
+
+  ComputedStyle* textContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCellText());
+
+  // Get the borders and padding for the text.
+  GetBorderPadding(textContext, bp);
+
+  RefPtr<nsFontMetrics> fm =
+      nsLayoutUtils::GetFontMetricsForComputedStyle(textContext, PresContext());
+  // Get the width of the text itself
+  nscoord width = nsLayoutUtils::AppUnitWidthOfStringBidi(cellText, this, *fm,
+                                                          *aRenderingContext);
+  nscoord totalTextWidth = width + bp.left + bp.right;
+  aDesiredSize += totalTextWidth;
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::IsCellCropped(int32_t aRow, nsTreeColumn* aCol,
+                                        bool* _retval) {
+  nscoord currentSize, desiredSize;
+  nsresult rv;
+
+  if (!aCol) return NS_ERROR_INVALID_ARG;
+
+  UniquePtr<gfxContext> rc = PresShell()->CreateReferenceRenderingContext();
+
+  rv = GetCellWidth(aRow, aCol, rc.get(), desiredSize, currentSize);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  *_retval = desiredSize > currentSize;
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::CreateTimer(const LookAndFeel::IntID aID,
+                                      nsTimerCallbackFunc aFunc, int32_t aType,
+                                      nsITimer** aTimer, const char* aName) {
+  // Get the delay from the look and feel service.
+  int32_t delay = LookAndFeel::GetInt(aID, 0);
+
+  nsCOMPtr<nsITimer> timer;
+
+  // Create a new timer only if the delay is greater than zero.
+  // Zero value means that this feature is completely disabled.
+  if (delay > 0) {
+    MOZ_TRY_VAR(timer,
+                NS_NewTimerWithFuncCallback(
+                    aFunc, this, delay, aType, aName,
+                    mContent->OwnerDoc()->EventTargetFor(TaskCategory::Other)));
+  }
+
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::RowCountChanged(int32_t aIndex, int32_t aCount) {
+  if (aCount == 0 || !mView) {
+    return NS_OK;  // Nothing to do.
+  }
+
+#ifdef ACCESSIBILITY
+  if (GetAccService()) {
+    FireRowCountChangedEvent(aIndex, aCount);
+  }
+#endif  // #ifdef ACCESSIBILITY
+
+  AutoWeakFrame weakFrame(this);
+
+  // Adjust our selection.
+  if (nsCOMPtr<nsITreeSelection> sel = GetSelection()) {
+    sel->AdjustSelection(aIndex, aCount);
+  }
+
+  NS_ENSURE_STATE(weakFrame.IsAlive());
+
+  if (mUpdateBatchNest) return NS_OK;
+
+  mRowCount += aCount;
+#ifdef DEBUG
+  int32_t rowCount = mRowCount;
+  mView->GetRowCount(&rowCount);
+  NS_ASSERTION(
+      rowCount == mRowCount,
+      "row count did not change by the amount suggested, check caller");
+#endif
+
+  int32_t count = Abs(aCount);
+  int32_t last = LastVisibleRow();
+  if (aIndex >= mTopRowIndex && aIndex <= last) InvalidateRange(aIndex, last);
+
+  ScrollParts parts = GetScrollParts();
+
+  if (mTopRowIndex == 0) {
+    // Just update the scrollbar and return.
+    FullScrollbarsUpdate(false);
+    return NS_OK;
+  }
+
+  bool needsInvalidation = false;
+  // Adjust our top row index.
+  if (aCount > 0) {
+    if (mTopRowIndex > aIndex) {
+      // Rows came in above us.  Augment the top row index.
+      mTopRowIndex += aCount;
+    }
+  } else if (aCount < 0) {
+    if (mTopRowIndex > aIndex + count - 1) {
+      // No need to invalidate. The remove happened
+      // completely above us (offscreen).
+      mTopRowIndex -= count;
+    } else if (mTopRowIndex >= aIndex) {
+      // This is a full-blown invalidate.
+      if (mTopRowIndex + mPageLength > mRowCount - 1) {
+        mTopRowIndex = std::max(0, mRowCount - 1 - mPageLength);
+      }
+      needsInvalidation = true;
+    }
+  }
+
+  FullScrollbarsUpdate(needsInvalidation);
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::BeginUpdateBatch() {
+  ++mUpdateBatchNest;
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::EndUpdateBatch() {
+  NS_ASSERTION(mUpdateBatchNest > 0, "badly nested update batch");
+
+  if (--mUpdateBatchNest != 0) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  if (!view) {
+    return NS_OK;
+  }
+
+  Invalidate();
+  int32_t countBeforeUpdate = mRowCount;
+  view->GetRowCount(&mRowCount);
+  if (countBeforeUpdate != mRowCount) {
+    if (mTopRowIndex + mPageLength > mRowCount - 1) {
+      mTopRowIndex = std::max(0, mRowCount - 1 - mPageLength);
+    }
+    FullScrollbarsUpdate(false);
+  }
+
+  return NS_OK;
+}
+
+void nsTreeBodyFrame::PrefillPropertyArray(int32_t aRowIndex,
+                                           nsTreeColumn* aCol) {
+  MOZ_ASSERT(!aCol || aCol->GetFrame(), "invalid column passed");
+  mScratchArray.Clear();
+
+  // focus
+  if (mFocused)
+    mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::focus);
+  else
+    mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::blur);
+
+  // sort
+  bool sorted = false;
+  mView->IsSorted(&sorted);
+  if (sorted) mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::sorted);
+
+  // drag session
+  if (mSlots && mSlots->mIsDragging)
+    mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::dragSession);
+
+  if (aRowIndex != -1) {
+    if (aRowIndex == mMouseOverRow)
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::hover);
+
+    nsCOMPtr<nsITreeSelection> selection = GetSelection();
+    if (selection) {
+      // selected
+      bool isSelected;
+      selection->IsSelected(aRowIndex, &isSelected);
+      if (isSelected)
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::selected);
+
+      // current
+      int32_t currentIndex;
+      selection->GetCurrentIndex(&currentIndex);
+      if (aRowIndex == currentIndex)
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::current);
+    }
+
+    // container or leaf
+    bool isContainer = false;
+    mView->IsContainer(aRowIndex, &isContainer);
+    if (isContainer) {
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::container);
+
+      // open or closed
+      bool isOpen = false;
+      mView->IsContainerOpen(aRowIndex, &isOpen);
+      if (isOpen)
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::open);
+      else
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::closed);
+    } else {
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::leaf);
+    }
+
+    // drop orientation
+    if (mSlots && mSlots->mDropAllowed && mSlots->mDropRow == aRowIndex) {
+      if (mSlots->mDropOrient == nsITreeView::DROP_BEFORE)
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::dropBefore);
+      else if (mSlots->mDropOrient == nsITreeView::DROP_ON)
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::dropOn);
+      else if (mSlots->mDropOrient == nsITreeView::DROP_AFTER)
+        mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::dropAfter);
+    }
+
+    // odd or even
+    if (aRowIndex % 2)
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::odd);
+    else
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::even);
+
+    XULTreeElement* tree = GetBaseElement();
+    if (tree && tree->HasAttr(kNameSpaceID_None, nsGkAtoms::editing)) {
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::editing);
+    }
+
+    // multiple columns
+    if (mColumns->GetColumnAt(1))
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::multicol);
+  }
+
+  if (aCol) {
+    mScratchArray.AppendElement(aCol->GetAtom());
+
+    if (aCol->IsPrimary())
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::primary);
+
+    if (aCol->GetType() == TreeColumn_Binding::TYPE_CHECKBOX) {
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::checkbox);
+
+      if (aRowIndex != -1) {
+        nsAutoString value;
+        mView->GetCellValue(aRowIndex, aCol, value);
+        if (value.EqualsLiteral("true"))
+          mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::checked);
+      }
+    }
+
+    // Read special properties from attributes on the column content node
+    if (aCol->mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::insertbefore,
+                                    nsGkAtoms::_true, eCaseMatters))
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::insertbefore);
+    if (aCol->mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::insertafter,
+                                    nsGkAtoms::_true, eCaseMatters))
+      mScratchArray.AppendElement((nsStaticAtom*)nsGkAtoms::insertafter);
+  }
+}
+
+nsITheme* nsTreeBodyFrame::GetTwistyRect(int32_t aRowIndex,
+                                         nsTreeColumn* aColumn,
+                                         nsRect& aImageRect,
+                                         nsRect& aTwistyRect,
+                                         nsPresContext* aPresContext,
+                                         ComputedStyle* aTwistyContext) {
+  // The twisty rect extends all the way to the end of the cell.  This is
+  // incorrect.  We need to determine the twisty rect's true width.  This is
+  // done by examining the ComputedStyle for a width first.  If it has one, we
+  // use that.  If it doesn't, we use the image's natural width. If the image
+  // hasn't loaded and if no width is specified, then we just bail. If there is
+  // a -moz-appearance involved, adjust the rect by the minimum widget size
+  // provided by the theme implementation.
+  aImageRect = GetImageSize(aRowIndex, aColumn, true, aTwistyContext);
+  if (aImageRect.height > aTwistyRect.height)
+    aImageRect.height = aTwistyRect.height;
+  if (aImageRect.width > aTwistyRect.width)
+    aImageRect.width = aTwistyRect.width;
+  else
+    aTwistyRect.width = aImageRect.width;
+
+  bool useTheme = false;
+  nsITheme* theme = nullptr;
+  StyleAppearance appearance =
+      aTwistyContext->StyleDisplay()->EffectiveAppearance();
+  if (appearance != StyleAppearance::None) {
+    theme = aPresContext->Theme();
+    if (theme->ThemeSupportsWidget(aPresContext, nullptr, appearance))
+      useTheme = true;
+  }
+
+  if (useTheme) {
+    LayoutDeviceIntSize minTwistySizePx =
+        theme->GetMinimumWidgetSize(aPresContext, this, appearance);
+
+    // GMWS() returns size in pixels, we need to convert it back to app units
+    nsSize minTwistySize;
+    minTwistySize.width =
+        aPresContext->DevPixelsToAppUnits(minTwistySizePx.width);
+    minTwistySize.height =
+        aPresContext->DevPixelsToAppUnits(minTwistySizePx.height);
+
+    if (aTwistyRect.width < minTwistySize.width) {
+      aTwistyRect.width = minTwistySize.width;
+    }
+  }
+
+  return useTheme ? theme : nullptr;
+}
+
+nsresult nsTreeBodyFrame::GetImage(int32_t aRowIndex, nsTreeColumn* aCol,
+                                   bool aUseContext,
+                                   ComputedStyle* aComputedStyle,
+                                   imgIContainer** aResult) {
+  *aResult = nullptr;
+
+  nsAutoString imageSrc;
+  mView->GetImageSrc(aRowIndex, aCol, imageSrc);
+  RefPtr<imgRequestProxy> styleRequest;
+  if (aUseContext || imageSrc.IsEmpty()) {
+    // Obtain the URL from the ComputedStyle.
+    styleRequest =
+        aComputedStyle->StyleList()->mListStyleImage.GetImageRequest();
+    if (!styleRequest) return NS_OK;
+    nsCOMPtr<nsIURI> uri;
+    styleRequest->GetURI(getter_AddRefs(uri));
+    nsAutoCString spec;
+    nsresult rv = uri->GetSpec(spec);
+    NS_ENSURE_SUCCESS(rv, rv);
+    CopyUTF8toUTF16(spec, imageSrc);
+  }
+
+  // Look the image up in our cache.
+  nsTreeImageCacheEntry entry;
+  if (mImageCache.Get(imageSrc, &entry)) {
+    // Find out if the image has loaded.
+    uint32_t status;
+    imgIRequest* imgReq = entry.request;
+    imgReq->GetImageStatus(&status);
+    imgReq->GetImage(aResult);  // We hand back the image here.  The GetImage
+                                // call addrefs *aResult.
+    bool animated = true;       // Assuming animated is the safe option
+
+    // We can only call GetAnimated if we're decoded
+    if (*aResult && (status & imgIRequest::STATUS_DECODE_COMPLETE))
+      (*aResult)->GetAnimated(&animated);
+
+    if ((!(status & imgIRequest::STATUS_LOAD_COMPLETE)) || animated) {
+      // We either aren't done loading, or we're animating. Add our row as a
+      // listener for invalidations.
+      nsCOMPtr<imgINotificationObserver> obs;
+      imgReq->GetNotificationObserver(getter_AddRefs(obs));
+
+      if (obs) {
+        static_cast<nsTreeImageListener*>(obs.get())->AddCell(aRowIndex, aCol);
+      }
+
+      return NS_OK;
+    }
+  }
+
+  if (!*aResult) {
+    // Create a new nsTreeImageListener object and pass it our row and column
+    // information.
+    nsTreeImageListener* listener = new nsTreeImageListener(this);
+    if (!listener) return NS_ERROR_OUT_OF_MEMORY;
+
+    mCreatedListeners.Insert(listener);
+
+    listener->AddCell(aRowIndex, aCol);
+    nsCOMPtr<imgINotificationObserver> imgNotificationObserver = listener;
+
+    Document* doc = mContent->GetComposedDoc();
+    if (!doc)
+      // The page is currently being torn down.  Why bother.
+      return NS_ERROR_FAILURE;
+
+    RefPtr<imgRequestProxy> imageRequest;
+    if (styleRequest) {
+      styleRequest->SyncClone(imgNotificationObserver, doc,
+                              getter_AddRefs(imageRequest));
+    } else {
+      nsCOMPtr<nsIURI> srcURI;
+      nsContentUtils::NewURIWithDocumentCharset(
+          getter_AddRefs(srcURI), imageSrc, doc, mContent->GetBaseURI());
+      if (!srcURI) return NS_ERROR_FAILURE;
+
+      auto referrerInfo = MakeRefPtr<mozilla::dom::ReferrerInfo>(*doc);
+
+      // XXXbz what's the origin principal for this stuff that comes from our
+      // view?  I guess we should assume that it's the node's principal...
+      nsresult rv = nsContentUtils::LoadImage(
+          srcURI, mContent, doc, mContent->NodePrincipal(), 0, referrerInfo,
+          imgNotificationObserver, nsIRequest::LOAD_NORMAL, u""_ns,
+          getter_AddRefs(imageRequest));
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      // NOTE(heycam): If it's an SVG image, and we need to want the image to
+      // able to respond to media query changes, it needs to be added to the
+      // document's ImageTracker.  For now, assume we don't need this.
+    }
+    listener->UnsuppressInvalidation();
+
+    if (!imageRequest) return NS_ERROR_FAILURE;
+
+    // We don't want discarding/decode-on-draw for xul images
+    imageRequest->StartDecoding(imgIContainer::FLAG_ASYNC_NOTIFY);
+    imageRequest->LockImage();
+
+    // In a case it was already cached.
+    imageRequest->GetImage(aResult);
+    nsTreeImageCacheEntry cacheEntry(imageRequest, imgNotificationObserver);
+    mImageCache.InsertOrUpdate(imageSrc, cacheEntry);
+  }
+  return NS_OK;
+}
+
+nsRect nsTreeBodyFrame::GetImageSize(int32_t aRowIndex, nsTreeColumn* aCol,
+                                     bool aUseContext,
+                                     ComputedStyle* aComputedStyle) {
+  // XXX We should respond to visibility rules for collapsed vs. hidden.
+
+  // This method returns the width of the twisty INCLUDING borders and padding.
+  // It first checks the ComputedStyle for a width.  If none is found, it tries
+  // to use the default image width for the twisty.  If no image is found, it
+  // defaults to border+padding.
+  nsRect r(0, 0, 0, 0);
+  nsMargin bp(0, 0, 0, 0);
+  GetBorderPadding(aComputedStyle, bp);
+  r.Inflate(bp);
+
+  // Now r contains our border+padding info.  We now need to get our width and
+  // height.
+  bool needWidth = false;
+  bool needHeight = false;
+
+  // We have to load image even though we already have a size.
+  // Don't change this, otherwise things start to go awry.
+  nsCOMPtr<imgIContainer> image;
+  GetImage(aRowIndex, aCol, aUseContext, aComputedStyle, getter_AddRefs(image));
+
+  const nsStylePosition* myPosition = aComputedStyle->StylePosition();
+  if (myPosition->mWidth.ConvertsToLength()) {
+    int32_t val = myPosition->mWidth.ToLength();
+    r.width += val;
+  } else {
+    needWidth = true;
+  }
+
+  if (myPosition->mHeight.ConvertsToLength()) {
+    int32_t val = myPosition->mHeight.ToLength();
+    r.height += val;
+  } else {
+    needHeight = true;
+  }
+
+  if (image) {
+    if (needWidth || needHeight) {
+      // Get the natural image size.
+
+      if (needWidth) {
+        // Get the size from the image.
+        nscoord width;
+        image->GetWidth(&width);
+        r.width += nsPresContext::CSSPixelsToAppUnits(width);
+      }
+
+      if (needHeight) {
+        nscoord height;
+        image->GetHeight(&height);
+        r.height += nsPresContext::CSSPixelsToAppUnits(height);
+      }
+    }
+  }
+
+  return r;
+}
+
+// GetImageDestSize returns the destination size of the image.
+// The width and height do not include borders and padding.
+// The width and height have not been adjusted to fit in the row height
+// or cell width.
+// The width and height reflect the destination size specified in CSS,
+// or the image region specified in CSS, or the natural size of the
+// image.
+// If only the destination width has been specified in CSS, the height is
+// calculated to maintain the aspect ratio of the image.
+// If only the destination height has been specified in CSS, the width is
+// calculated to maintain the aspect ratio of the image.
+nsSize nsTreeBodyFrame::GetImageDestSize(ComputedStyle* aComputedStyle,
+                                         imgIContainer* image) {
+  nsSize size(0, 0);
+
+  // We need to get the width and height.
+  bool needWidth = false;
+  bool needHeight = false;
+
+  // Get the style position to see if the CSS has specified the
+  // destination width/height.
+  const nsStylePosition* myPosition = aComputedStyle->StylePosition();
+
+  if (myPosition->mWidth.ConvertsToLength()) {
+    // CSS has specified the destination width.
+    size.width = myPosition->mWidth.ToLength();
+  } else {
+    // We'll need to get the width of the image/region.
+    needWidth = true;
+  }
+
+  if (myPosition->mHeight.ConvertsToLength()) {
+    // CSS has specified the destination height.
+    size.height = myPosition->mHeight.ToLength();
+  } else {
+    // We'll need to get the height of the image/region.
+    needHeight = true;
+  }
+
+  if (needWidth || needHeight) {
+    // We need to get the size of the image/region.
+    nsSize imageSize(0, 0);
+    if (image) {
+      nscoord width;
+      image->GetWidth(&width);
+      imageSize.width = nsPresContext::CSSPixelsToAppUnits(width);
+      nscoord height;
+      image->GetHeight(&height);
+      imageSize.height = nsPresContext::CSSPixelsToAppUnits(height);
+    }
+
+    if (needWidth) {
+      if (!needHeight && imageSize.height != 0) {
+        // The CSS specified the destination height, but not the destination
+        // width. We need to calculate the width so that we maintain the
+        // image's aspect ratio.
+        size.width = imageSize.width * size.height / imageSize.height;
+      } else {
+        size.width = imageSize.width;
+      }
+    }
+
+    if (needHeight) {
+      if (!needWidth && imageSize.width != 0) {
+        // The CSS specified the destination width, but not the destination
+        // height. We need to calculate the height so that we maintain the
+        // image's aspect ratio.
+        size.height = imageSize.height * size.width / imageSize.width;
+      } else {
+        size.height = imageSize.height;
+      }
+    }
+  }
+
+  return size;
+}
+
+// GetImageSourceRect returns the source rectangle of the image to be
+// displayed.
+// The width and height reflect the image region specified in CSS, or
+// the natural size of the image.
+// The width and height do not include borders and padding.
+// The width and height do not reflect the destination size specified
+// in CSS.
+nsRect nsTreeBodyFrame::GetImageSourceRect(ComputedStyle* aComputedStyle,
+                                           imgIContainer* image) {
+  if (!image) {
+    return nsRect();
+  }
+
+  nsRect r;
+  // Use the actual image size.
+  nscoord coord;
+  if (NS_SUCCEEDED(image->GetWidth(&coord))) {
+    r.width = nsPresContext::CSSPixelsToAppUnits(coord);
+  }
+  if (NS_SUCCEEDED(image->GetHeight(&coord))) {
+    r.height = nsPresContext::CSSPixelsToAppUnits(coord);
+  }
+  return r;
+}
+
+int32_t nsTreeBodyFrame::GetRowHeight() {
+  // Look up the correct height.  It is equal to the specified height
+  // + the specified margins.
+  mScratchArray.Clear();
+  ComputedStyle* rowContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeRow());
+  if (rowContext) {
+    const nsStylePosition* myPosition = rowContext->StylePosition();
+
+    nscoord minHeight = 0;
+    if (myPosition->mMinHeight.ConvertsToLength()) {
+      minHeight = myPosition->mMinHeight.ToLength();
+    }
+
+    nscoord height = 0;
+    if (myPosition->mHeight.ConvertsToLength()) {
+      height = myPosition->mHeight.ToLength();
+    }
+
+    if (height < minHeight) height = minHeight;
+
+    if (height > 0) {
+      height = nsPresContext::AppUnitsToIntCSSPixels(height);
+      height += height % 2;
+      height = nsPresContext::CSSPixelsToAppUnits(height);
+
+      // XXX Check box-sizing to determine if border/padding should augment the
+      // height Inflate the height by our margins.
+      nsRect rowRect(0, 0, 0, height);
+      nsMargin rowMargin;
+      rowContext->StyleMargin()->GetMargin(rowMargin);
+      rowRect.Inflate(rowMargin);
+      height = rowRect.height;
+      return height;
+    }
+  }
+
+  return nsPresContext::CSSPixelsToAppUnits(18);  // As good a default as any.
+}
+
+int32_t nsTreeBodyFrame::GetIndentation() {
+  // Look up the correct indentation.  It is equal to the specified indentation
+  // width.
+  mScratchArray.Clear();
+  ComputedStyle* indentContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeIndentation());
+  if (indentContext) {
+    const nsStylePosition* myPosition = indentContext->StylePosition();
+    if (myPosition->mWidth.ConvertsToLength()) {
+      return myPosition->mWidth.ToLength();
+    }
+  }
+
+  return nsPresContext::CSSPixelsToAppUnits(16);  // As good a default as any.
+}
+
+void nsTreeBodyFrame::CalcInnerBox() {
+  mInnerBox.SetRect(0, 0, mRect.width, mRect.height);
+  AdjustForBorderPadding(mComputedStyle, mInnerBox);
+}
+
+nscoord nsTreeBodyFrame::CalcHorzWidth(const ScrollParts& aParts) {
+  // Compute the adjustment to the last column. This varies depending on the
+  // visibility of the columnpicker and the scrollbar.
+  if (aParts.mColumnsFrame)
+    mAdjustWidth = mRect.width - aParts.mColumnsFrame->GetRect().width;
+  else
+    mAdjustWidth = 0;
+
+  nscoord width = 0;
+
+  // We calculate this from the scrollable frame, so that it
+  // properly covers all contingencies of what could be
+  // scrollable (columns, body, etc...)
+
+  if (aParts.mColumnsScrollFrame) {
+    width = aParts.mColumnsScrollFrame->GetScrollRange().width +
+            aParts.mColumnsScrollFrame->GetScrollPortRect().width;
+  }
+
+  // If no horz scrolling periphery is present, then just return our width
+  if (width == 0) width = mRect.width;
+
+  return width;
+}
+
+Maybe<nsIFrame::Cursor> nsTreeBodyFrame::GetCursor(const nsPoint& aPoint) {
+  // Check the GetScriptHandlingObject so we don't end up running code when
+  // the document is a zombie.
+  bool dummy;
+  if (mView && GetContent()->GetComposedDoc()->GetScriptHandlingObject(dummy)) {
+    int32_t row;
+    nsTreeColumn* col;
+    nsCSSAnonBoxPseudoStaticAtom* child;
+    GetCellAt(aPoint.x, aPoint.y, &row, &col, &child);
+
+    if (child) {
+      // Our scratch array is already prefilled.
+      RefPtr<ComputedStyle> childContext = GetPseudoComputedStyle(child);
+      StyleCursorKind kind = childContext->StyleUI()->Cursor().keyword;
+      if (kind == StyleCursorKind::Auto) {
+        kind = StyleCursorKind::Default;
+      }
+      return Some(
+          Cursor{kind, AllowCustomCursorImage::Yes, std::move(childContext)});
+    }
+  }
+  return SimpleXULLeafFrame::GetCursor(aPoint);
+}
+
+static uint32_t GetDropEffect(WidgetGUIEvent* aEvent) {
+  NS_ASSERTION(aEvent->mClass == eDragEventClass, "wrong event type");
+  WidgetDragEvent* dragEvent = aEvent->AsDragEvent();
+  nsContentUtils::SetDataTransferInEvent(dragEvent);
+
+  uint32_t action = 0;
+  if (dragEvent->mDataTransfer) {
+    action = dragEvent->mDataTransfer->DropEffectInt();
+  }
+  return action;
+}
+
+nsresult nsTreeBodyFrame::HandleEvent(nsPresContext* aPresContext,
+                                      WidgetGUIEvent* aEvent,
+                                      nsEventStatus* aEventStatus) {
+  if (aEvent->mMessage == eMouseOver || aEvent->mMessage == eMouseMove) {
+    nsPoint pt =
+        nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, RelativeTo{this});
+    int32_t xTwips = pt.x - mInnerBox.x;
+    int32_t yTwips = pt.y - mInnerBox.y;
+    int32_t newrow = GetRowAtInternal(xTwips, yTwips);
+    if (mMouseOverRow != newrow) {
+      // redraw the old and the new row
+      if (mMouseOverRow != -1) InvalidateRow(mMouseOverRow);
+      mMouseOverRow = newrow;
+      if (mMouseOverRow != -1) InvalidateRow(mMouseOverRow);
+    }
+  } else if (aEvent->mMessage == eMouseOut) {
+    if (mMouseOverRow != -1) {
+      InvalidateRow(mMouseOverRow);
+      mMouseOverRow = -1;
+    }
+  } else if (aEvent->mMessage == eDragEnter) {
+    if (!mSlots) {
+      mSlots = MakeUnique<Slots>();
+    }
+
+    // Cache several things we'll need throughout the course of our work. These
+    // will all get released on a drag exit.
+
+    if (mSlots->mTimer) {
+      mSlots->mTimer->Cancel();
+      mSlots->mTimer = nullptr;
+    }
+
+    // Cache the drag session.
+    mSlots->mIsDragging = true;
+    mSlots->mDropRow = -1;
+    mSlots->mDropOrient = -1;
+    mSlots->mDragAction = GetDropEffect(aEvent);
+  } else if (aEvent->mMessage == eDragOver) {
+    // The mouse is hovering over this tree. If we determine things are
+    // different from the last time, invalidate the drop feedback at the old
+    // position, query the view to see if the current location is droppable,
+    // and then invalidate the drop feedback at the new location if it is.
+    // The mouse may or may not have changed position from the last time
+    // we were called, so optimize out a lot of the extra notifications by
+    // checking if anything changed first. For drop feedback we use drop,
+    // dropBefore and dropAfter property.
+    if (!mView || !mSlots) {
+      return NS_OK;
+    }
+
+    // Save last values, we will need them.
+    int32_t lastDropRow = mSlots->mDropRow;
+    int16_t lastDropOrient = mSlots->mDropOrient;
+#ifndef XP_MACOSX
+    int16_t lastScrollLines = mSlots->mScrollLines;
+#endif
+
+    // Find out the current drag action
+    uint32_t lastDragAction = mSlots->mDragAction;
+    mSlots->mDragAction = GetDropEffect(aEvent);
+
+    // Compute the row mouse is over and the above/below/on state.
+    // Below we'll use this to see if anything changed.
+    // Also check if we want to auto-scroll.
+    ComputeDropPosition(aEvent, &mSlots->mDropRow, &mSlots->mDropOrient,
+                        &mSlots->mScrollLines);
+
+    // While we're here, handle tracking of scrolling during a drag.
+    if (mSlots->mScrollLines) {
+      if (mSlots->mDropAllowed) {
+        // Invalidate primary cell at old location.
+        mSlots->mDropAllowed = false;
+        InvalidateDropFeedback(lastDropRow, lastDropOrient);
+      }
+#ifdef XP_MACOSX
+      ScrollByLines(mSlots->mScrollLines);
+#else
+      if (!lastScrollLines) {
+        // Cancel any previously initialized timer.
+        if (mSlots->mTimer) {
+          mSlots->mTimer->Cancel();
+          mSlots->mTimer = nullptr;
+        }
+
+        // Set a timer to trigger the tree scrolling.
+        CreateTimer(LookAndFeel::IntID::TreeLazyScrollDelay, LazyScrollCallback,
+                    nsITimer::TYPE_ONE_SHOT, getter_AddRefs(mSlots->mTimer),
+                    "nsTreeBodyFrame::LazyScrollCallback");
+      }
+#endif
+      // Bail out to prevent spring loaded timer and feedback line settings.
+      return NS_OK;
+    }
+
+    // If changed from last time, invalidate primary cell at the old location
+    // and if allowed, invalidate primary cell at the new location. If nothing
+    // changed, just bail.
+    if (mSlots->mDropRow != lastDropRow ||
+        mSlots->mDropOrient != lastDropOrient ||
+        mSlots->mDragAction != lastDragAction) {
+      // Invalidate row at the old location.
+      if (mSlots->mDropAllowed) {
+        mSlots->mDropAllowed = false;
+        InvalidateDropFeedback(lastDropRow, lastDropOrient);
+      }
+
+      if (mSlots->mTimer) {
+        // Timer is active but for a different row than the current one, kill
+        // it.
+        mSlots->mTimer->Cancel();
+        mSlots->mTimer = nullptr;
+      }
+
+      if (mSlots->mDropRow >= 0) {
+        if (!mSlots->mTimer && mSlots->mDropOrient == nsITreeView::DROP_ON) {
+          // Either there wasn't a timer running or it was just killed above.
+          // If over a folder, start up a timer to open the folder.
+          bool isContainer = false;
+          mView->IsContainer(mSlots->mDropRow, &isContainer);
+          if (isContainer) {
+            bool isOpen = false;
+            mView->IsContainerOpen(mSlots->mDropRow, &isOpen);
+            if (!isOpen) {
+              // This node isn't expanded, set a timer to expand it.
+              CreateTimer(LookAndFeel::IntID::TreeOpenDelay, OpenCallback,
+                          nsITimer::TYPE_ONE_SHOT,
+                          getter_AddRefs(mSlots->mTimer),
+                          "nsTreeBodyFrame::OpenCallback");
+            }
+          }
+        }
+
+        // The dataTransfer was initialized by the call to GetDropEffect above.
+        bool canDropAtNewLocation = false;
+        mView->CanDrop(mSlots->mDropRow, mSlots->mDropOrient,
+                       aEvent->AsDragEvent()->mDataTransfer,
+                       &canDropAtNewLocation);
+
+        if (canDropAtNewLocation) {
+          // Invalidate row at the new location.
+          mSlots->mDropAllowed = canDropAtNewLocation;
+          InvalidateDropFeedback(mSlots->mDropRow, mSlots->mDropOrient);
+        }
+      }
+    }
+
+    // Indicate that the drop is allowed by preventing the default behaviour.
+    if (mSlots->mDropAllowed) *aEventStatus = nsEventStatus_eConsumeNoDefault;
+  } else if (aEvent->mMessage == eDrop) {
+    // this event was meant for another frame, so ignore it
+    if (!mSlots) return NS_OK;
+
+    // Tell the view where the drop happened.
+
+    // Remove the drop folder and all its parents from the array.
+    int32_t parentIndex;
+    nsresult rv = mView->GetParentIndex(mSlots->mDropRow, &parentIndex);
+    while (NS_SUCCEEDED(rv) && parentIndex >= 0) {
+      mSlots->mArray.RemoveElement(parentIndex);
+      rv = mView->GetParentIndex(parentIndex, &parentIndex);
+    }
+
+    NS_ASSERTION(aEvent->mClass == eDragEventClass, "wrong event type");
+    WidgetDragEvent* dragEvent = aEvent->AsDragEvent();
+    nsContentUtils::SetDataTransferInEvent(dragEvent);
+
+    mView->Drop(mSlots->mDropRow, mSlots->mDropOrient,
+                dragEvent->mDataTransfer);
+    mSlots->mDropRow = -1;
+    mSlots->mDropOrient = -1;
+    mSlots->mIsDragging = false;
+    *aEventStatus =
+        nsEventStatus_eConsumeNoDefault;  // already handled the drop
+  } else if (aEvent->mMessage == eDragExit) {
+    // this event was meant for another frame, so ignore it
+    if (!mSlots) return NS_OK;
+
+    // Clear out all our tracking vars.
+
+    if (mSlots->mDropAllowed) {
+      mSlots->mDropAllowed = false;
+      InvalidateDropFeedback(mSlots->mDropRow, mSlots->mDropOrient);
+    } else
+      mSlots->mDropAllowed = false;
+    mSlots->mIsDragging = false;
+    mSlots->mScrollLines = 0;
+    // If a drop is occuring, the exit event will fire just before the drop
+    // event, so don't reset mDropRow or mDropOrient as these fields are used
+    // by the drop event.
+    if (mSlots->mTimer) {
+      mSlots->mTimer->Cancel();
+      mSlots->mTimer = nullptr;
+    }
+
+    if (!mSlots->mArray.IsEmpty()) {
+      // Close all spring loaded folders except the drop folder.
+      CreateTimer(LookAndFeel::IntID::TreeCloseDelay, CloseCallback,
+                  nsITimer::TYPE_ONE_SHOT, getter_AddRefs(mSlots->mTimer),
+                  "nsTreeBodyFrame::CloseCallback");
+    }
+  }
+
+  return NS_OK;
+}
+
+namespace mozilla {
+
+class nsDisplayTreeBody final : public nsPaintedDisplayItem {
+ public:
+  nsDisplayTreeBody(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsPaintedDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayTreeBody);
+  }
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayTreeBody)
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayTreeBodyGeometry(this, aBuilder, IsWindowActive());
+  }
+
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    aBuilder->UnregisterThemeGeometry(this);
+    nsPaintedDisplayItem::Destroy(aBuilder);
+  }
+
+  bool IsWindowActive() const {
+    DocumentState docState =
+        mFrame->PresContext()->Document()->GetDocumentState();
+    return !docState.HasState(DocumentState::WINDOW_INACTIVE);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    auto geometry = static_cast<const nsDisplayTreeBodyGeometry*>(aGeometry);
+
+    if (IsWindowActive() != geometry->mWindowIsActive) {
+      bool snap;
+      aInvalidRegion->Or(*aInvalidRegion, GetBounds(aBuilder, &snap));
+    }
+
+    nsPaintedDisplayItem::ComputeInvalidationRegion(aBuilder, aGeometry,
+                                                    aInvalidRegion);
+  }
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    MOZ_ASSERT(aBuilder);
+    Unused << static_cast<nsTreeBodyFrame*>(mFrame)->PaintTreeBody(
+        *aCtx, GetPaintRect(aBuilder, aCtx), ToReferenceFrame(), aBuilder);
+  }
+
+  NS_DISPLAY_DECL_NAME("XULTreeBody", TYPE_XUL_TREE_BODY)
+
+  nsRect GetComponentAlphaBounds(
+      nsDisplayListBuilder* aBuilder) const override {
+    bool snap;
+    return GetBounds(aBuilder, &snap);
+  }
+};
+
+}  // namespace mozilla
+
+#ifdef XP_MACOSX
+static bool IsInSourceList(nsIFrame* aFrame) {
+  for (nsIFrame* frame = aFrame; frame;
+       frame = nsLayoutUtils::GetCrossDocParentFrameInProcess(frame)) {
+    if (frame->StyleDisplay()->EffectiveAppearance() ==
+        StyleAppearance::MozMacSourceList) {
+      return true;
+    }
+  }
+  return false;
+}
+#endif
+
+// Painting routines
+void nsTreeBodyFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                                       const nsDisplayListSet& aLists) {
+  // REVIEW: why did we paint if we were collapsed? that makes no sense!
+  if (!IsVisibleForPainting()) return;  // We're invisible.  Don't paint.
+
+  // Handles painting our background, border, and outline.
+  SimpleXULLeafFrame::BuildDisplayList(aBuilder, aLists);
+
+  // Bail out now if there's no view or we can't run script because the
+  // document is a zombie
+  if (!mView || !GetContent()->GetComposedDoc()->GetWindow()) return;
+
+  nsDisplayItem* item = MakeDisplayItem<nsDisplayTreeBody>(aBuilder, this);
+  aLists.Content()->AppendToTop(item);
+
+#ifdef XP_MACOSX
+  XULTreeElement* tree = GetBaseElement();
+  nsIFrame* treeFrame = tree ? tree->GetPrimaryFrame() : nullptr;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  nsCOMPtr<nsITreeSelection> selection = GetSelection();
+  nsITheme* theme = PresContext()->Theme();
+  // On Mac, we support native theming of selected rows. On 10.10 and higher,
+  // this means applying vibrancy which require us to register the theme
+  // geometrics for the row. In order to make the vibrancy effect to work
+  // properly, we also need an ancestor frame to be themed as a source list.
+  if (selection && theme && IsInSourceList(treeFrame)) {
+    // Loop through our onscreen rows. If the row is selected and a
+    // -moz-appearance is provided, RegisterThemeGeometry might be necessary.
+    const auto end = std::min(mRowCount, LastVisibleRow() + 1);
+    for (auto i = FirstVisibleRow(); i < end; i++) {
+      bool isSelected;
+      selection->IsSelected(i, &isSelected);
+      if (isSelected) {
+        PrefillPropertyArray(i, nullptr);
+        nsAutoString properties;
+        view->GetRowProperties(i, properties);
+        nsTreeUtils::TokenizeProperties(properties, mScratchArray);
+        ComputedStyle* rowContext =
+            GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeRow());
+        auto appearance = rowContext->StyleDisplay()->EffectiveAppearance();
+        if (appearance != StyleAppearance::None) {
+          if (theme->ThemeSupportsWidget(PresContext(), this, appearance)) {
+            nsITheme::ThemeGeometryType type =
+                theme->ThemeGeometryTypeForWidget(this, appearance);
+            if (type != nsITheme::eThemeGeometryTypeUnknown) {
+              nsRect rowRect(mInnerBox.x,
+                             mInnerBox.y + mRowHeight * (i - FirstVisibleRow()),
+                             mInnerBox.width, mRowHeight);
+              aBuilder->RegisterThemeGeometry(
+                  type, item,
+                  LayoutDeviceIntRect::FromUnknownRect(
+                      (rowRect + aBuilder->ToReferenceFrame(this))
+                          .ToNearestPixels(
+                              PresContext()->AppUnitsPerDevPixel())));
+            }
+          }
+        }
+      }
+    }
+  }
+#endif
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintTreeBody(gfxContext& aRenderingContext,
+                                             const nsRect& aDirtyRect,
+                                             nsPoint aPt,
+                                             nsDisplayListBuilder* aBuilder) {
+  // Update our available height and our page count.
+  CalcInnerBox();
+
+  DrawTarget* drawTarget = aRenderingContext.GetDrawTarget();
+
+  aRenderingContext.Save();
+  aRenderingContext.Clip(NSRectToSnappedRect(
+      mInnerBox + aPt, PresContext()->AppUnitsPerDevPixel(), *drawTarget));
+  int32_t oldPageCount = mPageLength;
+  if (!mHasFixedRowCount) {
+    mPageLength =
+        (mRowHeight > 0) ? (mInnerBox.height / mRowHeight) : mRowCount;
+  }
+
+  if (oldPageCount != mPageLength ||
+      mHorzWidth != CalcHorzWidth(GetScrollParts())) {
+    // Schedule a ResizeReflow that will update our info properly.
+    PresShell()->FrameNeedsReflow(this, IntrinsicDirty::FrameAndAncestors,
+                                  NS_FRAME_IS_DIRTY);
+  }
+#ifdef DEBUG
+  int32_t rowCount = mRowCount;
+  mView->GetRowCount(&rowCount);
+  NS_WARNING_ASSERTION(mRowCount == rowCount, "row count changed unexpectedly");
+#endif
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  // Loop through our columns and paint them (e.g., for sorting).  This is only
+  // relevant when painting backgrounds, since columns contain no content.
+  // Content is contained in the rows.
+  for (nsTreeColumn* currCol = mColumns->GetFirstColumn(); currCol;
+       currCol = currCol->GetNext()) {
+    nsRect colRect;
+    nsresult rv =
+        currCol->GetRect(this, mInnerBox.y, mInnerBox.height, &colRect);
+    // Don't paint hidden columns.
+    if (NS_FAILED(rv) || colRect.width == 0) continue;
+
+    if (OffsetForHorzScroll(colRect, false)) {
+      nsRect dirtyRect;
+      colRect += aPt;
+      if (dirtyRect.IntersectRect(aDirtyRect, colRect)) {
+        result &= PaintColumn(currCol, colRect, PresContext(),
+                              aRenderingContext, aDirtyRect);
+      }
+    }
+  }
+  // Loop through our on-screen rows.
+  for (int32_t i = mTopRowIndex;
+       i < mRowCount && i <= mTopRowIndex + mPageLength; i++) {
+    nsRect rowRect(mInnerBox.x, mInnerBox.y + mRowHeight * (i - mTopRowIndex),
+                   mInnerBox.width, mRowHeight);
+    nsRect dirtyRect;
+    if (dirtyRect.IntersectRect(aDirtyRect, rowRect + aPt) &&
+        rowRect.y < (mInnerBox.y + mInnerBox.height)) {
+      result &= PaintRow(i, rowRect + aPt, PresContext(), aRenderingContext,
+                         aDirtyRect, aPt, aBuilder);
+    }
+  }
+
+  if (mSlots && mSlots->mDropAllowed &&
+      (mSlots->mDropOrient == nsITreeView::DROP_BEFORE ||
+       mSlots->mDropOrient == nsITreeView::DROP_AFTER)) {
+    nscoord yPos = mInnerBox.y +
+                   mRowHeight * (mSlots->mDropRow - mTopRowIndex) -
+                   mRowHeight / 2;
+    nsRect feedbackRect(mInnerBox.x, yPos, mInnerBox.width, mRowHeight);
+    if (mSlots->mDropOrient == nsITreeView::DROP_AFTER)
+      feedbackRect.y += mRowHeight;
+
+    nsRect dirtyRect;
+    feedbackRect += aPt;
+    if (dirtyRect.IntersectRect(aDirtyRect, feedbackRect)) {
+      result &= PaintDropFeedback(feedbackRect, PresContext(),
+                                  aRenderingContext, aDirtyRect, aPt);
+    }
+  }
+  aRenderingContext.Restore();
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintColumn(nsTreeColumn* aColumn,
+                                           const nsRect& aColumnRect,
+                                           nsPresContext* aPresContext,
+                                           gfxContext& aRenderingContext,
+                                           const nsRect& aDirtyRect) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  // Now obtain the properties for our cell.
+  PrefillPropertyArray(-1, aColumn);
+  nsAutoString properties;
+
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  view->GetColumnProperties(aColumn, properties);
+  nsTreeUtils::TokenizeProperties(properties, mScratchArray);
+
+  // Resolve style for the column.  It contains all the info we need to lay
+  // ourselves out and to paint.
+  ComputedStyle* colContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeColumn());
+
+  // Obtain the margins for the cell and then deflate our rect by that
+  // amount.  The cell is assumed to be contained within the deflated rect.
+  nsRect colRect(aColumnRect);
+  nsMargin colMargin;
+  colContext->StyleMargin()->GetMargin(colMargin);
+  colRect.Deflate(colMargin);
+
+  return PaintBackgroundLayer(colContext, aPresContext, aRenderingContext,
+                              colRect, aDirtyRect);
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintRow(int32_t aRowIndex,
+                                        const nsRect& aRowRect,
+                                        nsPresContext* aPresContext,
+                                        gfxContext& aRenderingContext,
+                                        const nsRect& aDirtyRect, nsPoint aPt,
+                                        nsDisplayListBuilder* aBuilder) {
+  // We have been given a rect for our row.  We treat this row like a full-blown
+  // frame, meaning that it can have borders, margins, padding, and a
+  // background.
+
+  // Without a view, we have no data. Check for this up front.
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  if (!view) {
+    return ImgDrawResult::SUCCESS;
+  }
+
+  nsresult rv;
+
+  // Now obtain the properties for our row.
+  // XXX Automatically fill in the following props: open, closed, container,
+  // leaf, selected, focused
+  PrefillPropertyArray(aRowIndex, nullptr);
+
+  nsAutoString properties;
+  view->GetRowProperties(aRowIndex, properties);
+  nsTreeUtils::TokenizeProperties(properties, mScratchArray);
+
+  // Resolve style for the row.  It contains all the info we need to lay
+  // ourselves out and to paint.
+  ComputedStyle* rowContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeRow());
+
+  // Obtain the margins for the row and then deflate our rect by that
+  // amount.  The row is assumed to be contained within the deflated rect.
+  nsRect rowRect(aRowRect);
+  nsMargin rowMargin;
+  rowContext->StyleMargin()->GetMargin(rowMargin);
+  rowRect.Deflate(rowMargin);
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  // Paint our borders and background for our row rect.
+  nsITheme* theme = nullptr;
+  auto appearance = rowContext->StyleDisplay()->EffectiveAppearance();
+  if (appearance != StyleAppearance::None) {
+    theme = aPresContext->Theme();
+  }
+
+  if (theme && theme->ThemeSupportsWidget(aPresContext, nullptr, appearance)) {
+    nsRect dirty;
+    dirty.IntersectRect(rowRect, aDirtyRect);
+    theme->DrawWidgetBackground(&aRenderingContext, this, appearance, rowRect,
+                                dirty);
+  } else {
+    result &= PaintBackgroundLayer(rowContext, aPresContext, aRenderingContext,
+                                   rowRect, aDirtyRect);
+  }
+
+  // Adjust the rect for its border and padding.
+  nsRect originalRowRect = rowRect;
+  AdjustForBorderPadding(rowContext, rowRect);
+
+  bool isSeparator = false;
+  view->IsSeparator(aRowIndex, &isSeparator);
+  if (isSeparator) {
+    // The row is a separator.
+
+    nscoord primaryX = rowRect.x;
+    nsTreeColumn* primaryCol = mColumns->GetPrimaryColumn();
+    if (primaryCol) {
+      // Paint the primary cell.
+      nsRect cellRect;
+      rv = primaryCol->GetRect(this, rowRect.y, rowRect.height, &cellRect);
+      if (NS_FAILED(rv)) {
+        MOZ_ASSERT_UNREACHABLE("primary column is invalid");
+        return result;
+      }
+
+      if (OffsetForHorzScroll(cellRect, false)) {
+        cellRect.x += aPt.x;
+        nsRect dirtyRect;
+        nsRect checkRect(cellRect.x, originalRowRect.y, cellRect.width,
+                         originalRowRect.height);
+        if (dirtyRect.IntersectRect(aDirtyRect, checkRect)) {
+          result &=
+              PaintCell(aRowIndex, primaryCol, cellRect, aPresContext,
+                        aRenderingContext, aDirtyRect, primaryX, aPt, aBuilder);
+        }
+      }
+
+      // Paint the left side of the separator.
+      nscoord currX;
+      nsTreeColumn* previousCol = primaryCol->GetPrevious();
+      if (previousCol) {
+        nsRect prevColRect;
+        rv = previousCol->GetRect(this, 0, 0, &prevColRect);
+        if (NS_SUCCEEDED(rv)) {
+          currX = (prevColRect.x - mHorzPosition) + prevColRect.width + aPt.x;
+        } else {
+          MOZ_ASSERT_UNREACHABLE(
+              "The column before the primary column is "
+              "invalid");
+          currX = rowRect.x;
+        }
+      } else {
+        currX = rowRect.x;
+      }
+
+      int32_t level;
+      view->GetLevel(aRowIndex, &level);
+      if (level == 0) currX += mIndentation;
+
+      if (currX > rowRect.x) {
+        nsRect separatorRect(rowRect);
+        separatorRect.width -= rowRect.x + rowRect.width - currX;
+        result &= PaintSeparator(aRowIndex, separatorRect, aPresContext,
+                                 aRenderingContext, aDirtyRect);
+      }
+    }
+
+    // Paint the right side (whole) separator.
+    nsRect separatorRect(rowRect);
+    if (primaryX > rowRect.x) {
+      separatorRect.width -= primaryX - rowRect.x;
+      separatorRect.x += primaryX - rowRect.x;
+    }
+    result &= PaintSeparator(aRowIndex, separatorRect, aPresContext,
+                             aRenderingContext, aDirtyRect);
+  } else {
+    // Now loop over our cells. Only paint a cell if it intersects with our
+    // dirty rect.
+    for (nsTreeColumn* currCol = mColumns->GetFirstColumn(); currCol;
+         currCol = currCol->GetNext()) {
+      nsRect cellRect;
+      rv = currCol->GetRect(this, rowRect.y, rowRect.height, &cellRect);
+      // Don't paint cells in hidden columns.
+      if (NS_FAILED(rv) || cellRect.width == 0) continue;
+
+      if (OffsetForHorzScroll(cellRect, false)) {
+        cellRect.x += aPt.x;
+
+        // for primary columns, use the row's vertical size so that the
+        // lines get drawn properly
+        nsRect checkRect = cellRect;
+        if (currCol->IsPrimary())
+          checkRect = nsRect(cellRect.x, originalRowRect.y, cellRect.width,
+                             originalRowRect.height);
+
+        nsRect dirtyRect;
+        nscoord dummy;
+        if (dirtyRect.IntersectRect(aDirtyRect, checkRect))
+          result &=
+              PaintCell(aRowIndex, currCol, cellRect, aPresContext,
+                        aRenderingContext, aDirtyRect, dummy, aPt, aBuilder);
+      }
+    }
+  }
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintSeparator(int32_t aRowIndex,
+                                              const nsRect& aSeparatorRect,
+                                              nsPresContext* aPresContext,
+                                              gfxContext& aRenderingContext,
+                                              const nsRect& aDirtyRect) {
+  // Resolve style for the separator.
+  ComputedStyle* separatorContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeSeparator());
+  bool useTheme = false;
+  nsITheme* theme = nullptr;
+  StyleAppearance appearance =
+      separatorContext->StyleDisplay()->EffectiveAppearance();
+  if (appearance != StyleAppearance::None) {
+    theme = aPresContext->Theme();
+    if (theme->ThemeSupportsWidget(aPresContext, nullptr, appearance))
+      useTheme = true;
+  }
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  // use -moz-appearance if provided.
+  if (useTheme) {
+    nsRect dirty;
+    dirty.IntersectRect(aSeparatorRect, aDirtyRect);
+    theme->DrawWidgetBackground(&aRenderingContext, this, appearance,
+                                aSeparatorRect, dirty);
+  } else {
+    const nsStylePosition* stylePosition = separatorContext->StylePosition();
+
+    // Obtain the height for the separator or use the default value.
+    nscoord height;
+    if (stylePosition->mHeight.ConvertsToLength()) {
+      height = stylePosition->mHeight.ToLength();
+    } else {
+      // Use default height 2px.
+      height = nsPresContext::CSSPixelsToAppUnits(2);
+    }
+
+    // Obtain the margins for the separator and then deflate our rect by that
+    // amount. The separator is assumed to be contained within the deflated
+    // rect.
+    nsRect separatorRect(aSeparatorRect.x, aSeparatorRect.y,
+                         aSeparatorRect.width, height);
+    nsMargin separatorMargin;
+    separatorContext->StyleMargin()->GetMargin(separatorMargin);
+    separatorRect.Deflate(separatorMargin);
+
+    // Center the separator.
+    separatorRect.y += (aSeparatorRect.height - height) / 2;
+
+    result &=
+        PaintBackgroundLayer(separatorContext, aPresContext, aRenderingContext,
+                             separatorRect, aDirtyRect);
+  }
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintCell(
+    int32_t aRowIndex, nsTreeColumn* aColumn, const nsRect& aCellRect,
+    nsPresContext* aPresContext, gfxContext& aRenderingContext,
+    const nsRect& aDirtyRect, nscoord& aCurrX, nsPoint aPt,
+    nsDisplayListBuilder* aBuilder) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  // Now obtain the properties for our cell.
+  // XXX Automatically fill in the following props: open, closed, container,
+  // leaf, selected, focused, and the col ID.
+  PrefillPropertyArray(aRowIndex, aColumn);
+  nsAutoString properties;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  view->GetCellProperties(aRowIndex, aColumn, properties);
+  nsTreeUtils::TokenizeProperties(properties, mScratchArray);
+
+  // Resolve style for the cell.  It contains all the info we need to lay
+  // ourselves out and to paint.
+  ComputedStyle* cellContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCell());
+
+  bool isRTL = StyleVisibility()->mDirection == StyleDirection::Rtl;
+
+  // Obtain the margins for the cell and then deflate our rect by that
+  // amount.  The cell is assumed to be contained within the deflated rect.
+  nsRect cellRect(aCellRect);
+  nsMargin cellMargin;
+  cellContext->StyleMargin()->GetMargin(cellMargin);
+  cellRect.Deflate(cellMargin);
+
+  // Paint our borders and background for our row rect.
+  ImgDrawResult result = PaintBackgroundLayer(
+      cellContext, aPresContext, aRenderingContext, cellRect, aDirtyRect);
+
+  // Adjust the rect for its border and padding.
+  AdjustForBorderPadding(cellContext, cellRect);
+
+  nscoord currX = cellRect.x;
+  nscoord remainingWidth = cellRect.width;
+
+  // Now we paint the contents of the cells.
+  // Directionality of the tree determines the order in which we paint.
+  // StyleDirection::Ltr means paint from left to right.
+  // StyleDirection::Rtl means paint from right to left.
+
+  if (aColumn->IsPrimary()) {
+    // If we're the primary column, we need to indent and paint the twisty and
+    // any connecting lines between siblings.
+
+    int32_t level;
+    view->GetLevel(aRowIndex, &level);
+
+    if (!isRTL) currX += mIndentation * level;
+    remainingWidth -= mIndentation * level;
+
+    // Resolve the style to use for the connecting lines.
+    ComputedStyle* lineContext =
+        GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeLine());
+
+    if (mIndentation && level &&
+        lineContext->StyleVisibility()->IsVisibleOrCollapsed()) {
+      // Paint the thread lines.
+
+      // Get the size of the twisty. We don't want to paint the twisty
+      // before painting of connecting lines since it would paint lines over
+      // the twisty. But we need to leave a place for it.
+      ComputedStyle* twistyContext =
+          GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeTwisty());
+
+      nsRect imageSize;
+      nsRect twistyRect(aCellRect);
+      GetTwistyRect(aRowIndex, aColumn, imageSize, twistyRect, aPresContext,
+                    twistyContext);
+
+      nsMargin twistyMargin;
+      twistyContext->StyleMargin()->GetMargin(twistyMargin);
+      twistyRect.Inflate(twistyMargin);
+
+      const nsStyleBorder* borderStyle = lineContext->StyleBorder();
+      // Resolve currentcolor values against the treeline context
+      nscolor color = borderStyle->mBorderLeftColor.CalcColor(*lineContext);
+      ColorPattern colorPatt(ToDeviceColor(color));
+
+      StyleBorderStyle style = borderStyle->GetBorderStyle(eSideLeft);
+      StrokeOptions strokeOptions;
+      nsLayoutUtils::InitDashPattern(strokeOptions, style);
+
+      nscoord srcX = currX + twistyRect.width - mIndentation / 2;
+      nscoord lineY = (aRowIndex - mTopRowIndex) * mRowHeight + aPt.y;
+
+      DrawTarget* drawTarget = aRenderingContext.GetDrawTarget();
+      nsPresContext* pc = PresContext();
+
+      // Don't paint off our cell.
+      if (srcX <= cellRect.x + cellRect.width) {
+        nscoord destX = currX + twistyRect.width;
+        if (destX > cellRect.x + cellRect.width)
+          destX = cellRect.x + cellRect.width;
+        if (isRTL) {
+          srcX = currX + remainingWidth - (srcX - cellRect.x);
+          destX = currX + remainingWidth - (destX - cellRect.x);
+        }
+        Point p1(pc->AppUnitsToGfxUnits(srcX),
+                 pc->AppUnitsToGfxUnits(lineY + mRowHeight / 2));
+        Point p2(pc->AppUnitsToGfxUnits(destX),
+                 pc->AppUnitsToGfxUnits(lineY + mRowHeight / 2));
+        SnapLineToDevicePixelsForStroking(p1, p2, *drawTarget,
+                                          strokeOptions.mLineWidth);
+        drawTarget->StrokeLine(p1, p2, colorPatt, strokeOptions);
+      }
+
+      int32_t currentParent = aRowIndex;
+      for (int32_t i = level; i > 0; i--) {
+        if (srcX <= cellRect.x + cellRect.width) {
+          // Paint full vertical line only if we have next sibling.
+          bool hasNextSibling;
+          view->HasNextSibling(currentParent, aRowIndex, &hasNextSibling);
+          if (hasNextSibling || i == level) {
+            Point p1(pc->AppUnitsToGfxUnits(srcX),
+                     pc->AppUnitsToGfxUnits(lineY));
+            Point p2;
+            p2.x = pc->AppUnitsToGfxUnits(srcX);
+
+            if (hasNextSibling)
+              p2.y = pc->AppUnitsToGfxUnits(lineY + mRowHeight);
+            else if (i == level)
+              p2.y = pc->AppUnitsToGfxUnits(lineY + mRowHeight / 2);
+
+            SnapLineToDevicePixelsForStroking(p1, p2, *drawTarget,
+                                              strokeOptions.mLineWidth);
+            drawTarget->StrokeLine(p1, p2, colorPatt, strokeOptions);
+          }
+        }
+
+        int32_t parent;
+        if (NS_FAILED(view->GetParentIndex(currentParent, &parent)) ||
+            parent < 0)
+          break;
+        currentParent = parent;
+        srcX -= mIndentation;
+      }
+    }
+
+    // Always leave space for the twisty.
+    nsRect twistyRect(currX, cellRect.y, remainingWidth, cellRect.height);
+    result &= PaintTwisty(aRowIndex, aColumn, twistyRect, aPresContext,
+                          aRenderingContext, aDirtyRect, remainingWidth, currX);
+  }
+
+  // Now paint the icon for our cell.
+  nsRect iconRect(currX, cellRect.y, remainingWidth, cellRect.height);
+  nsRect dirtyRect;
+  if (dirtyRect.IntersectRect(aDirtyRect, iconRect)) {
+    result &= PaintImage(aRowIndex, aColumn, iconRect, aPresContext,
+                         aRenderingContext, aDirtyRect, remainingWidth, currX,
+                         aBuilder);
+  }
+
+  // Now paint our element, but only if we aren't a cycler column.
+  // XXX until we have the ability to load images, allow the view to
+  // insert text into cycler columns...
+  if (!aColumn->IsCycler()) {
+    nsRect elementRect(currX, cellRect.y, remainingWidth, cellRect.height);
+    nsRect dirtyRect;
+    if (dirtyRect.IntersectRect(aDirtyRect, elementRect)) {
+      switch (aColumn->GetType()) {
+        case TreeColumn_Binding::TYPE_TEXT:
+          result &= PaintText(aRowIndex, aColumn, elementRect, aPresContext,
+                              aRenderingContext, aDirtyRect, currX);
+          break;
+        case TreeColumn_Binding::TYPE_CHECKBOX:
+          result &= PaintCheckbox(aRowIndex, aColumn, elementRect, aPresContext,
+                                  aRenderingContext, aDirtyRect);
+          break;
+      }
+    }
+  }
+
+  aCurrX = currX;
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintTwisty(
+    int32_t aRowIndex, nsTreeColumn* aColumn, const nsRect& aTwistyRect,
+    nsPresContext* aPresContext, gfxContext& aRenderingContext,
+    const nsRect& aDirtyRect, nscoord& aRemainingWidth, nscoord& aCurrX) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  bool isRTL = StyleVisibility()->mDirection == StyleDirection::Rtl;
+  nscoord rightEdge = aCurrX + aRemainingWidth;
+  // Paint the twisty, but only if we are a non-empty container.
+  bool shouldPaint = false;
+  bool isContainer = false;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  view->IsContainer(aRowIndex, &isContainer);
+  if (isContainer) {
+    bool isContainerEmpty = false;
+    view->IsContainerEmpty(aRowIndex, &isContainerEmpty);
+    if (!isContainerEmpty) shouldPaint = true;
+  }
+
+  // Resolve style for the twisty.
+  ComputedStyle* twistyContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeTwisty());
+
+  // Obtain the margins for the twisty and then deflate our rect by that
+  // amount.  The twisty is assumed to be contained within the deflated rect.
+  nsRect twistyRect(aTwistyRect);
+  nsMargin twistyMargin;
+  twistyContext->StyleMargin()->GetMargin(twistyMargin);
+  twistyRect.Deflate(twistyMargin);
+
+  nsRect imageSize;
+  nsITheme* theme = GetTwistyRect(aRowIndex, aColumn, imageSize, twistyRect,
+                                  aPresContext, twistyContext);
+
+  // Subtract out the remaining width.  This is done even when we don't actually
+  // paint a twisty in this cell, so that cells in different rows still line up.
+  nsRect copyRect(twistyRect);
+  copyRect.Inflate(twistyMargin);
+  aRemainingWidth -= copyRect.width;
+  if (!isRTL) aCurrX += copyRect.width;
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  if (shouldPaint) {
+    // Paint our borders and background for our image rect.
+    result &= PaintBackgroundLayer(twistyContext, aPresContext,
+                                   aRenderingContext, twistyRect, aDirtyRect);
+
+    if (theme) {
+      if (isRTL) twistyRect.x = rightEdge - twistyRect.width;
+      // yeah, I know it says we're drawing a background, but a twisty is really
+      // a fg object since it doesn't have anything that gecko would want to
+      // draw over it. Besides, we have to prevent imagelib from drawing it.
+      nsRect dirty;
+      dirty.IntersectRect(twistyRect, aDirtyRect);
+      theme->DrawWidgetBackground(
+          &aRenderingContext, this,
+          twistyContext->StyleDisplay()->EffectiveAppearance(), twistyRect,
+          dirty);
+    } else {
+      // Time to paint the twisty.
+      // Adjust the rect for its border and padding.
+      nsMargin bp(0, 0, 0, 0);
+      GetBorderPadding(twistyContext, bp);
+      twistyRect.Deflate(bp);
+      if (isRTL) twistyRect.x = rightEdge - twistyRect.width;
+      imageSize.Deflate(bp);
+
+      // Get the image for drawing.
+      nsCOMPtr<imgIContainer> image;
+      GetImage(aRowIndex, aColumn, true, twistyContext, getter_AddRefs(image));
+      if (image) {
+        nsPoint anchorPoint = twistyRect.TopLeft();
+
+        // Center the image. XXX Obey vertical-align style prop?
+        if (imageSize.height < twistyRect.height) {
+          anchorPoint.y += (twistyRect.height - imageSize.height) / 2;
+        }
+
+        // Apply context paint if applicable
+        SVGImageContext svgContext;
+        SVGImageContext::MaybeStoreContextPaint(svgContext, *aPresContext,
+                                                *twistyContext, image);
+
+        // Paint the image.
+        result &= nsLayoutUtils::DrawSingleUnscaledImage(
+            aRenderingContext, aPresContext, image, SamplingFilter::POINT,
+            anchorPoint, &aDirtyRect, svgContext, imgIContainer::FLAG_NONE,
+            &imageSize);
+      }
+    }
+  }
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintImage(
+    int32_t aRowIndex, nsTreeColumn* aColumn, const nsRect& aImageRect,
+    nsPresContext* aPresContext, gfxContext& aRenderingContext,
+    const nsRect& aDirtyRect, nscoord& aRemainingWidth, nscoord& aCurrX,
+    nsDisplayListBuilder* aBuilder) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  bool isRTL = StyleVisibility()->mDirection == StyleDirection::Rtl;
+  nscoord rightEdge = aCurrX + aRemainingWidth;
+  // Resolve style for the image.
+  ComputedStyle* imageContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeImage());
+
+  // Obtain the margins for the image and then deflate our rect by that
+  // amount.  The image is assumed to be contained within the deflated rect.
+  nsRect imageRect(aImageRect);
+  nsMargin imageMargin;
+  imageContext->StyleMargin()->GetMargin(imageMargin);
+  imageRect.Deflate(imageMargin);
+
+  // Get the image.
+  nsCOMPtr<imgIContainer> image;
+  GetImage(aRowIndex, aColumn, false, imageContext, getter_AddRefs(image));
+
+  // Get the image destination size.
+  nsSize imageDestSize = GetImageDestSize(imageContext, image);
+  if (!imageDestSize.width || !imageDestSize.height) {
+    return ImgDrawResult::SUCCESS;
+  }
+
+  // Get the borders and padding.
+  nsMargin bp(0, 0, 0, 0);
+  GetBorderPadding(imageContext, bp);
+
+  // destRect will be passed as the aDestRect argument in the DrawImage method.
+  // Start with the imageDestSize width and height.
+  nsRect destRect(0, 0, imageDestSize.width, imageDestSize.height);
+  // Inflate destRect for borders and padding so that we can compare/adjust
+  // with respect to imageRect.
+  destRect.Inflate(bp);
+
+  // The destRect width and height have not been adjusted to fit within the
+  // cell width and height.
+  // We must adjust the width even if image is null, because the width is used
+  // to update the aRemainingWidth and aCurrX values.
+  // Since the height isn't used unless the image is not null, we will adjust
+  // the height inside the if (image) block below.
+
+  if (destRect.width > imageRect.width) {
+    // The destRect is too wide to fit within the cell width.
+    // Adjust destRect width to fit within the cell width.
+    destRect.width = imageRect.width;
+  } else {
+    // The cell is wider than the destRect.
+    // In a cycler column, the image is centered horizontally.
+    if (!aColumn->IsCycler()) {
+      // If this column is not a cycler, we won't center the image horizontally.
+      // We adjust the imageRect width so that the image is placed at the start
+      // of the cell.
+      imageRect.width = destRect.width;
+    }
+  }
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  if (image) {
+    if (isRTL) imageRect.x = rightEdge - imageRect.width;
+    // Paint our borders and background for our image rect
+    result &= PaintBackgroundLayer(imageContext, aPresContext,
+                                   aRenderingContext, imageRect, aDirtyRect);
+
+    // The destRect x and y have not been set yet. Let's do that now.
+    // Initially, we use the imageRect x and y.
+    destRect.x = imageRect.x;
+    destRect.y = imageRect.y;
+
+    if (destRect.width < imageRect.width) {
+      // The destRect width is smaller than the cell width.
+      // Center the image horizontally in the cell.
+      // Adjust the destRect x accordingly.
+      destRect.x += (imageRect.width - destRect.width) / 2;
+    }
+
+    // Now it's time to adjust the destRect height to fit within the cell
+    // height.
+    if (destRect.height > imageRect.height) {
+      // The destRect height is larger than the cell height.
+      // Adjust destRect height to fit within the cell height.
+      destRect.height = imageRect.height;
+    } else if (destRect.height < imageRect.height) {
+      // The destRect height is smaller than the cell height.
+      // Center the image vertically in the cell.
+      // Adjust the destRect y accordingly.
+      destRect.y += (imageRect.height - destRect.height) / 2;
+    }
+
+    // It's almost time to paint the image.
+    // Deflate destRect for the border and padding.
+    destRect.Deflate(bp);
+
+    // Compute the area where our whole image would be mapped, to get the
+    // desired subregion onto our actual destRect:
+    nsRect wholeImageDest;
+    CSSIntSize rawImageCSSIntSize;
+    if (NS_SUCCEEDED(image->GetWidth(&rawImageCSSIntSize.width)) &&
+        NS_SUCCEEDED(image->GetHeight(&rawImageCSSIntSize.height))) {
+      // Get the image source rectangle - the rectangle containing the part of
+      // the image that we are going to display.  sourceRect will be passed as
+      // the aSrcRect argument in the DrawImage method.
+      nsRect sourceRect = GetImageSourceRect(imageContext, image);
+
+      // Let's say that the image is 100 pixels tall and that the CSS has
+      // specified that the destination height should be 50 pixels tall. Let's
+      // say that the cell height is only 20 pixels. So, in those 20 visible
+      // pixels, we want to see the top 20/50ths of the image.  So, the
+      // sourceRect.height should be 100 * 20 / 50, which is 40 pixels.
+      // Essentially, we are scaling the image as dictated by the CSS
+      // destination height and width, and we are then clipping the scaled
+      // image by the cell width and height.
+      nsSize rawImageSize(CSSPixel::ToAppUnits(rawImageCSSIntSize));
+      wholeImageDest = nsLayoutUtils::GetWholeImageDestination(
+          rawImageSize, sourceRect, nsRect(destRect.TopLeft(), imageDestSize));
+    } else {
+      // GetWidth/GetHeight failed, so we can't easily map a subregion of the
+      // source image onto the destination area.
+      // * If this happens with a RasterImage, it probably means the image is
+      // in an error state, and we shouldn't draw anything. Hence, we leave
+      // wholeImageDest as an empty rect (its initial state).
+      // * If this happens with a VectorImage, it probably means the image has
+      // no explicit width or height attribute -- but we can still proceed and
+      // just treat the destination area as our whole SVG image area. Hence, we
+      // set wholeImageDest to the full destRect.
+      if (image->GetType() == imgIContainer::TYPE_VECTOR) {
+        wholeImageDest = destRect;
+      }
+    }
+
+    const auto* styleEffects = imageContext->StyleEffects();
+    gfxGroupForBlendAutoSaveRestore autoGroupForBlend(&aRenderingContext);
+    if (!styleEffects->IsOpaque()) {
+      autoGroupForBlend.PushGroupForBlendBack(gfxContentType::COLOR_ALPHA,
+                                              styleEffects->mOpacity);
+    }
+
+    uint32_t drawFlags = aBuilder && aBuilder->UseHighQualityScaling()
+                             ? imgIContainer::FLAG_HIGH_QUALITY_SCALING
+                             : imgIContainer::FLAG_NONE;
+    result &= nsLayoutUtils::DrawImage(
+        aRenderingContext, imageContext, aPresContext, image,
+        nsLayoutUtils::GetSamplingFilterForFrame(this), wholeImageDest,
+        destRect, destRect.TopLeft(), aDirtyRect, drawFlags);
+  }
+
+  // Update the aRemainingWidth and aCurrX values.
+  imageRect.Inflate(imageMargin);
+  aRemainingWidth -= imageRect.width;
+  if (!isRTL) {
+    aCurrX += imageRect.width;
+  }
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintText(
+    int32_t aRowIndex, nsTreeColumn* aColumn, const nsRect& aTextRect,
+    nsPresContext* aPresContext, gfxContext& aRenderingContext,
+    const nsRect& aDirtyRect, nscoord& aCurrX) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  bool isRTL = StyleVisibility()->mDirection == StyleDirection::Rtl;
+
+  // Now obtain the text for our cell.
+  nsAutoString text;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  view->GetCellText(aRowIndex, aColumn, text);
+
+  // We're going to paint this text so we need to ensure bidi is enabled if
+  // necessary
+  CheckTextForBidi(text);
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  if (text.Length() == 0) {
+    // Don't paint an empty string. XXX What about background/borders? Still
+    // paint?
+    return result;
+  }
+
+  int32_t appUnitsPerDevPixel = PresContext()->AppUnitsPerDevPixel();
+  DrawTarget* drawTarget = aRenderingContext.GetDrawTarget();
+
+  // Resolve style for the text.  It contains all the info we need to lay
+  // ourselves out and to paint.
+  ComputedStyle* textContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCellText());
+
+  // Obtain the margins for the text and then deflate our rect by that
+  // amount.  The text is assumed to be contained within the deflated rect.
+  nsRect textRect(aTextRect);
+  nsMargin textMargin;
+  textContext->StyleMargin()->GetMargin(textMargin);
+  textRect.Deflate(textMargin);
+
+  // Adjust the rect for its border and padding.
+  nsMargin bp(0, 0, 0, 0);
+  GetBorderPadding(textContext, bp);
+  textRect.Deflate(bp);
+
+  // Compute our text size.
+  RefPtr<nsFontMetrics> fontMet =
+      nsLayoutUtils::GetFontMetricsForComputedStyle(textContext, PresContext());
+
+  nscoord height = fontMet->MaxHeight();
+  nscoord baseline = fontMet->MaxAscent();
+
+  // Center the text. XXX Obey vertical-align style prop?
+  if (height < textRect.height) {
+    textRect.y += (textRect.height - height) / 2;
+    textRect.height = height;
+  }
+
+  // Set our font.
+  AdjustForCellText(text, aRowIndex, aColumn, aRenderingContext, *fontMet,
+                    textRect);
+  textRect.Inflate(bp);
+
+  // Subtract out the remaining width.
+  if (!isRTL) aCurrX += textRect.width + textMargin.LeftRight();
+
+  result &= PaintBackgroundLayer(textContext, aPresContext, aRenderingContext,
+                                 textRect, aDirtyRect);
+
+  // Time to paint our text.
+  textRect.Deflate(bp);
+
+  // Set our color.
+  ColorPattern color(ToDeviceColor(textContext->StyleText()->mColor));
+
+  // Draw decorations.
+  StyleTextDecorationLine decorations =
+      textContext->StyleTextReset()->mTextDecorationLine;
+
+  nscoord offset;
+  nscoord size;
+  if (decorations & (StyleTextDecorationLine::OVERLINE |
+                     StyleTextDecorationLine::UNDERLINE)) {
+    fontMet->GetUnderline(offset, size);
+    if (decorations & StyleTextDecorationLine::OVERLINE) {
+      nsRect r(textRect.x, textRect.y, textRect.width, size);
+      Rect devPxRect = NSRectToSnappedRect(r, appUnitsPerDevPixel, *drawTarget);
+      drawTarget->FillRect(devPxRect, color);
+    }
+    if (decorations & StyleTextDecorationLine::UNDERLINE) {
+      nsRect r(textRect.x, textRect.y + baseline - offset, textRect.width,
+               size);
+      Rect devPxRect = NSRectToSnappedRect(r, appUnitsPerDevPixel, *drawTarget);
+      drawTarget->FillRect(devPxRect, color);
+    }
+  }
+  if (decorations & StyleTextDecorationLine::LINE_THROUGH) {
+    fontMet->GetStrikeout(offset, size);
+    nsRect r(textRect.x, textRect.y + baseline - offset, textRect.width, size);
+    Rect devPxRect = NSRectToSnappedRect(r, appUnitsPerDevPixel, *drawTarget);
+    drawTarget->FillRect(devPxRect, color);
+  }
+  ComputedStyle* cellContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCell());
+
+  const auto* styleEffects = textContext->StyleEffects();
+  gfxGroupForBlendAutoSaveRestore autoGroupForBlend(&aRenderingContext);
+  if (!styleEffects->IsOpaque()) {
+    autoGroupForBlend.PushGroupForBlendBack(gfxContentType::COLOR_ALPHA,
+                                            styleEffects->mOpacity);
+  }
+
+  aRenderingContext.SetColor(
+      sRGBColor::FromABGR(textContext->StyleText()->mColor.ToColor()));
+  nsLayoutUtils::DrawString(
+      this, *fontMet, &aRenderingContext, text.get(), text.Length(),
+      textRect.TopLeft() + nsPoint(0, baseline), cellContext);
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintCheckbox(int32_t aRowIndex,
+                                             nsTreeColumn* aColumn,
+                                             const nsRect& aCheckboxRect,
+                                             nsPresContext* aPresContext,
+                                             gfxContext& aRenderingContext,
+                                             const nsRect& aDirtyRect) {
+  MOZ_ASSERT(aColumn && aColumn->GetFrame(), "invalid column passed");
+
+  // Resolve style for the checkbox.
+  ComputedStyle* checkboxContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeCheckbox());
+
+  nscoord rightEdge = aCheckboxRect.XMost();
+
+  // Obtain the margins for the checkbox and then deflate our rect by that
+  // amount.  The checkbox is assumed to be contained within the deflated rect.
+  nsRect checkboxRect(aCheckboxRect);
+  nsMargin checkboxMargin;
+  checkboxContext->StyleMargin()->GetMargin(checkboxMargin);
+  checkboxRect.Deflate(checkboxMargin);
+
+  nsRect imageSize = GetImageSize(aRowIndex, aColumn, true, checkboxContext);
+
+  if (imageSize.height > checkboxRect.height) {
+    imageSize.height = checkboxRect.height;
+  }
+  if (imageSize.width > checkboxRect.width) {
+    imageSize.width = checkboxRect.width;
+  }
+
+  if (StyleVisibility()->mDirection == StyleDirection::Rtl) {
+    checkboxRect.x = rightEdge - checkboxRect.width;
+  }
+
+  // Paint our borders and background for our image rect.
+  ImgDrawResult result =
+      PaintBackgroundLayer(checkboxContext, aPresContext, aRenderingContext,
+                           checkboxRect, aDirtyRect);
+
+  // Time to paint the checkbox.
+  // Adjust the rect for its border and padding.
+  nsMargin bp(0, 0, 0, 0);
+  GetBorderPadding(checkboxContext, bp);
+  checkboxRect.Deflate(bp);
+
+  // Get the image for drawing.
+  nsCOMPtr<imgIContainer> image;
+  GetImage(aRowIndex, aColumn, true, checkboxContext, getter_AddRefs(image));
+  if (image) {
+    nsPoint pt = checkboxRect.TopLeft();
+
+    if (imageSize.height < checkboxRect.height) {
+      pt.y += (checkboxRect.height - imageSize.height) / 2;
+    }
+
+    if (imageSize.width < checkboxRect.width) {
+      pt.x += (checkboxRect.width - imageSize.width) / 2;
+    }
+
+    // Apply context paint if applicable
+    SVGImageContext svgContext;
+    SVGImageContext::MaybeStoreContextPaint(svgContext, *aPresContext,
+                                            *checkboxContext, image);
+    // Paint the image.
+    result &= nsLayoutUtils::DrawSingleUnscaledImage(
+        aRenderingContext, aPresContext, image, SamplingFilter::POINT, pt,
+        &aDirtyRect, svgContext, imgIContainer::FLAG_NONE, &imageSize);
+  }
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintDropFeedback(
+    const nsRect& aDropFeedbackRect, nsPresContext* aPresContext,
+    gfxContext& aRenderingContext, const nsRect& aDirtyRect, nsPoint aPt) {
+  // Paint the drop feedback in between rows.
+
+  nscoord currX;
+
+  // Adjust for the primary cell.
+  nsTreeColumn* primaryCol = mColumns->GetPrimaryColumn();
+
+  if (primaryCol) {
+#ifdef DEBUG
+    nsresult rv =
+#endif
+        primaryCol->GetXInTwips(this, &currX);
+    NS_ASSERTION(NS_SUCCEEDED(rv), "primary column is invalid?");
+
+    currX += aPt.x - mHorzPosition;
+  } else {
+    currX = aDropFeedbackRect.x;
+  }
+
+  PrefillPropertyArray(mSlots->mDropRow, primaryCol);
+
+  // Resolve the style to use for the drop feedback.
+  ComputedStyle* feedbackContext =
+      GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeDropFeedback());
+
+  ImgDrawResult result = ImgDrawResult::SUCCESS;
+
+  // Paint only if it is visible.
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  if (feedbackContext->StyleVisibility()->IsVisibleOrCollapsed()) {
+    int32_t level;
+    view->GetLevel(mSlots->mDropRow, &level);
+
+    // If our previous or next row has greater level use that for
+    // correct visual indentation.
+    if (mSlots->mDropOrient == nsITreeView::DROP_BEFORE) {
+      if (mSlots->mDropRow > 0) {
+        int32_t previousLevel;
+        view->GetLevel(mSlots->mDropRow - 1, &previousLevel);
+        if (previousLevel > level) level = previousLevel;
+      }
+    } else {
+      if (mSlots->mDropRow < mRowCount - 1) {
+        int32_t nextLevel;
+        view->GetLevel(mSlots->mDropRow + 1, &nextLevel);
+        if (nextLevel > level) level = nextLevel;
+      }
+    }
+
+    currX += mIndentation * level;
+
+    if (primaryCol) {
+      ComputedStyle* twistyContext =
+          GetPseudoComputedStyle(nsCSSAnonBoxes::mozTreeTwisty());
+      nsRect imageSize;
+      nsRect twistyRect;
+      GetTwistyRect(mSlots->mDropRow, primaryCol, imageSize, twistyRect,
+                    aPresContext, twistyContext);
+      nsMargin twistyMargin;
+      twistyContext->StyleMargin()->GetMargin(twistyMargin);
+      twistyRect.Inflate(twistyMargin);
+      currX += twistyRect.width;
+    }
+
+    const nsStylePosition* stylePosition = feedbackContext->StylePosition();
+
+    // Obtain the width for the drop feedback or use default value.
+    nscoord width;
+    if (stylePosition->mWidth.ConvertsToLength()) {
+      width = stylePosition->mWidth.ToLength();
+    } else {
+      // Use default width 50px.
+      width = nsPresContext::CSSPixelsToAppUnits(50);
+    }
+
+    // Obtain the height for the drop feedback or use default value.
+    nscoord height;
+    if (stylePosition->mHeight.ConvertsToLength()) {
+      height = stylePosition->mHeight.ToLength();
+    } else {
+      // Use default height 2px.
+      height = nsPresContext::CSSPixelsToAppUnits(2);
+    }
+
+    // Obtain the margins for the drop feedback and then deflate our rect
+    // by that amount.
+    nsRect feedbackRect(currX, aDropFeedbackRect.y, width, height);
+    nsMargin margin;
+    feedbackContext->StyleMargin()->GetMargin(margin);
+    feedbackRect.Deflate(margin);
+
+    feedbackRect.y += (aDropFeedbackRect.height - height) / 2;
+
+    // Finally paint the drop feedback.
+    result &= PaintBackgroundLayer(feedbackContext, aPresContext,
+                                   aRenderingContext, feedbackRect, aDirtyRect);
+  }
+
+  return result;
+}
+
+ImgDrawResult nsTreeBodyFrame::PaintBackgroundLayer(
+    ComputedStyle* aComputedStyle, nsPresContext* aPresContext,
+    gfxContext& aRenderingContext, const nsRect& aRect,
+    const nsRect& aDirtyRect) {
+  const nsStyleBorder* myBorder = aComputedStyle->StyleBorder();
+  nsCSSRendering::PaintBGParams params =
+      nsCSSRendering::PaintBGParams::ForAllLayers(
+          *aPresContext, aDirtyRect, aRect, this,
+          nsCSSRendering::PAINTBG_SYNC_DECODE_IMAGES);
+  ImgDrawResult result = nsCSSRendering::PaintStyleImageLayerWithSC(
+      params, aRenderingContext, aComputedStyle, *myBorder);
+
+  result &= nsCSSRendering::PaintBorderWithStyleBorder(
+      aPresContext, aRenderingContext, this, aDirtyRect, aRect, *myBorder,
+      mComputedStyle, PaintBorderFlags::SyncDecodeImages);
+
+  nsCSSRendering::PaintNonThemedOutline(aPresContext, aRenderingContext, this,
+                                        aDirtyRect, aRect, aComputedStyle);
+
+  return result;
+}
+
+// Scrolling
+nsresult nsTreeBodyFrame::EnsureRowIsVisible(int32_t aRow) {
+  ScrollParts parts = GetScrollParts();
+  nsresult rv = EnsureRowIsVisibleInternal(parts, aRow);
+  NS_ENSURE_SUCCESS(rv, rv);
+  UpdateScrollbars(parts);
+  return rv;
+}
+
+nsresult nsTreeBodyFrame::EnsureRowIsVisibleInternal(const ScrollParts& aParts,
+                                                     int32_t aRow) {
+  if (!mView || !mPageLength) {
+    return NS_OK;
+  }
+
+  if (mTopRowIndex <= aRow && mTopRowIndex + mPageLength > aRow) return NS_OK;
+
+  if (aRow < mTopRowIndex)
+    ScrollToRowInternal(aParts, aRow);
+  else {
+    // Bring it just on-screen.
+    int32_t distance = aRow - (mTopRowIndex + mPageLength) + 1;
+    ScrollToRowInternal(aParts, mTopRowIndex + distance);
+  }
+
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::EnsureCellIsVisible(int32_t aRow,
+                                              nsTreeColumn* aCol) {
+  if (!aCol) return NS_ERROR_INVALID_ARG;
+
+  ScrollParts parts = GetScrollParts();
+
+  nscoord result = -1;
+  nsresult rv;
+
+  nscoord columnPos;
+  rv = aCol->GetXInTwips(this, &columnPos);
+  if (NS_FAILED(rv)) return rv;
+
+  nscoord columnWidth;
+  rv = aCol->GetWidthInTwips(this, &columnWidth);
+  if (NS_FAILED(rv)) return rv;
+
+  // If the start of the column is before the
+  // start of the horizontal view, then scroll
+  if (columnPos < mHorzPosition) result = columnPos;
+  // If the end of the column is past the end of
+  // the horizontal view, then scroll
+  else if ((columnPos + columnWidth) > (mHorzPosition + mInnerBox.width))
+    result = ((columnPos + columnWidth) - (mHorzPosition + mInnerBox.width)) +
+             mHorzPosition;
+
+  if (result != -1) {
+    rv = ScrollHorzInternal(parts, result);
+    if (NS_FAILED(rv)) return rv;
+  }
+
+  rv = EnsureRowIsVisibleInternal(parts, aRow);
+  NS_ENSURE_SUCCESS(rv, rv);
+  UpdateScrollbars(parts);
+  return rv;
+}
+
+void nsTreeBodyFrame::ScrollToRow(int32_t aRow) {
+  ScrollParts parts = GetScrollParts();
+  ScrollToRowInternal(parts, aRow);
+  UpdateScrollbars(parts);
+}
+
+nsresult nsTreeBodyFrame::ScrollToRowInternal(const ScrollParts& aParts,
+                                              int32_t aRow) {
+  ScrollInternal(aParts, aRow);
+
+  return NS_OK;
+}
+
+void nsTreeBodyFrame::ScrollByLines(int32_t aNumLines) {
+  if (!mView) {
+    return;
+  }
+  int32_t newIndex = mTopRowIndex + aNumLines;
+  ScrollToRow(newIndex);
+}
+
+void nsTreeBodyFrame::ScrollByPages(int32_t aNumPages) {
+  if (!mView) {
+    return;
+  }
+  int32_t newIndex = mTopRowIndex + aNumPages * mPageLength;
+  ScrollToRow(newIndex);
+}
+
+nsresult nsTreeBodyFrame::ScrollInternal(const ScrollParts& aParts,
+                                         int32_t aRow) {
+  if (!mView) {
+    return NS_OK;
+  }
+
+  // Note that we may be "over scrolled" at this point; that is the
+  // current mTopRowIndex may be larger than mRowCount - mPageLength.
+  // This can happen when items are removed for example. (bug 1085050)
+
+  int32_t maxTopRowIndex = std::max(0, mRowCount - mPageLength);
+  aRow = mozilla::clamped(aRow, 0, maxTopRowIndex);
+  if (aRow == mTopRowIndex) {
+    return NS_OK;
+  }
+  mTopRowIndex = aRow;
+  Invalidate();
+  PostScrollEvent();
+  return NS_OK;
+}
+
+nsresult nsTreeBodyFrame::ScrollHorzInternal(const ScrollParts& aParts,
+                                             int32_t aPosition) {
+  if (!mView || !aParts.mColumnsScrollFrame || !aParts.mHScrollbar)
+    return NS_OK;
+
+  if (aPosition == mHorzPosition) return NS_OK;
+
+  if (aPosition < 0 || aPosition > mHorzWidth) return NS_OK;
+
+  nsRect bounds = aParts.mColumnsFrame->GetRect();
+  if (aPosition > (mHorzWidth - bounds.width))
+    aPosition = mHorzWidth - bounds.width;
+
+  mHorzPosition = aPosition;
+
+  Invalidate();
+
+  // Update the column scroll view
+  AutoWeakFrame weakFrame(this);
+  aParts.mColumnsScrollFrame->ScrollTo(nsPoint(mHorzPosition, 0),
+                                       ScrollMode::Instant);
+  if (!weakFrame.IsAlive()) {
+    return NS_ERROR_FAILURE;
+  }
+  // And fire off an event about it all
+  PostScrollEvent();
+  return NS_OK;
+}
+
+void nsTreeBodyFrame::ScrollByPage(nsScrollbarFrame* aScrollbar,
+                                   int32_t aDirection,
+                                   ScrollSnapFlags aSnapFlags) {
+  // CSS Scroll Snapping is not enabled for XUL, aSnap is ignored
+  MOZ_ASSERT(aScrollbar != nullptr);
+  ScrollByPages(aDirection);
+}
+
+void nsTreeBodyFrame::ScrollByWhole(nsScrollbarFrame* aScrollbar,
+                                    int32_t aDirection,
+                                    ScrollSnapFlags aSnapFlags) {
+  // CSS Scroll Snapping is not enabled for XUL, aSnap is ignored
+  MOZ_ASSERT(aScrollbar != nullptr);
+  int32_t newIndex = aDirection < 0 ? 0 : mTopRowIndex;
+  ScrollToRow(newIndex);
+}
+
+void nsTreeBodyFrame::ScrollByLine(nsScrollbarFrame* aScrollbar,
+                                   int32_t aDirection,
+                                   ScrollSnapFlags aSnapFlags) {
+  // CSS Scroll Snapping is not enabled for XUL, aSnap is ignored
+  MOZ_ASSERT(aScrollbar != nullptr);
+  ScrollByLines(aDirection);
+}
+
+void nsTreeBodyFrame::ScrollByUnit(
+    nsScrollbarFrame* aScrollbar, ScrollMode aMode, int32_t aDirection,
+    ScrollUnit aUnit, ScrollSnapFlags aSnapFlags /* = Disabled */) {
+  MOZ_ASSERT_UNREACHABLE("Can't get here, we pass false to MoveToNewPosition");
+}
+
+void nsTreeBodyFrame::RepeatButtonScroll(nsScrollbarFrame* aScrollbar) {
+  ScrollParts parts = GetScrollParts();
+  int32_t increment = aScrollbar->GetIncrement();
+  int32_t direction = 0;
+  if (increment < 0) {
+    direction = -1;
+  } else if (increment > 0) {
+    direction = 1;
+  }
+  bool isHorizontal = aScrollbar->IsHorizontal();
+
+  AutoWeakFrame weakFrame(this);
+  if (isHorizontal) {
+    int32_t curpos = aScrollbar->MoveToNewPosition(
+        nsScrollbarFrame::ImplementsScrollByUnit::No);
+    if (weakFrame.IsAlive()) {
+      ScrollHorzInternal(parts, curpos);
+    }
+  } else {
+    ScrollToRowInternal(parts, mTopRowIndex + direction);
+  }
+
+  if (weakFrame.IsAlive() && mScrollbarActivity) {
+    mScrollbarActivity->ActivityOccurred();
+  }
+  if (weakFrame.IsAlive()) {
+    UpdateScrollbars(parts);
+  }
+}
+
+void nsTreeBodyFrame::ThumbMoved(nsScrollbarFrame* aScrollbar, nscoord aOldPos,
+                                 nscoord aNewPos) {
+  ScrollParts parts = GetScrollParts();
+
+  if (aOldPos == aNewPos) return;
+
+  AutoWeakFrame weakFrame(this);
+
+  // Vertical Scrollbar
+  if (parts.mVScrollbar == aScrollbar) {
+    nscoord rh = nsPresContext::AppUnitsToIntCSSPixels(mRowHeight);
+    nscoord newIndex = nsPresContext::AppUnitsToIntCSSPixels(aNewPos);
+    nscoord newrow = (rh > 0) ? (newIndex / rh) : 0;
+    ScrollInternal(parts, newrow);
+    // Horizontal Scrollbar
+  } else if (parts.mHScrollbar == aScrollbar) {
+    int32_t newIndex = nsPresContext::AppUnitsToIntCSSPixels(aNewPos);
+    ScrollHorzInternal(parts, newIndex);
+  }
+  if (weakFrame.IsAlive()) {
+    UpdateScrollbars(parts);
+  }
+}
+
+// The style cache.
+ComputedStyle* nsTreeBodyFrame::GetPseudoComputedStyle(
+    nsCSSAnonBoxPseudoStaticAtom* aPseudoElement) {
+  return mStyleCache.GetComputedStyle(PresContext(), mContent, mComputedStyle,
+                                      aPseudoElement, mScratchArray);
+}
+
+XULTreeElement* nsTreeBodyFrame::GetBaseElement() {
+  if (!mTree) {
+    nsIFrame* parent = GetParent();
+    while (parent) {
+      nsIContent* content = parent->GetContent();
+      if (content && content->IsXULElement(nsGkAtoms::tree)) {
+        mTree = XULTreeElement::FromNodeOrNull(content->AsElement());
+        break;
+      }
+
+      parent = parent->GetInFlowParent();
+    }
+  }
+
+  return mTree;
+}
+
+nsresult nsTreeBodyFrame::ClearStyleAndImageCaches() {
+  mStyleCache.Clear();
+  CancelImageRequests();
+  mImageCache.Clear();
+  return NS_OK;
+}
+
+void nsTreeBodyFrame::RemoveImageCacheEntry(int32_t aRowIndex,
+                                            nsTreeColumn* aCol) {
+  nsAutoString imageSrc;
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  if (NS_FAILED(view->GetImageSrc(aRowIndex, aCol, imageSrc))) {
+    return;
+  }
+  nsTreeImageCacheEntry entry;
+  if (!mImageCache.Get(imageSrc, &entry)) {
+    return;
+  }
+  nsLayoutUtils::DeregisterImageRequest(PresContext(), entry.request, nullptr);
+  entry.request->UnlockImage();
+  entry.request->CancelAndForgetObserver(NS_BINDING_ABORTED);
+  mImageCache.Remove(imageSrc);
+}
+
+/* virtual */
+void nsTreeBodyFrame::DidSetComputedStyle(ComputedStyle* aOldComputedStyle) {
+  SimpleXULLeafFrame::DidSetComputedStyle(aOldComputedStyle);
+
+  // Clear the style cache; the pointers are no longer even valid
+  mStyleCache.Clear();
+  // XXX The following is hacky, but it's not incorrect,
+  // and appears to fix a few bugs with style changes, like text zoom and
+  // dpi changes
+  mIndentation = GetIndentation();
+  mRowHeight = GetRowHeight();
+}
+
+bool nsTreeBodyFrame::OffsetForHorzScroll(nsRect& rect, bool clip) {
+  rect.x -= mHorzPosition;
+
+  // Scrolled out before
+  if (rect.XMost() <= mInnerBox.x) return false;
+
+  // Scrolled out after
+  if (rect.x > mInnerBox.XMost()) return false;
+
+  if (clip) {
+    nscoord leftEdge = std::max(rect.x, mInnerBox.x);
+    nscoord rightEdge = std::min(rect.XMost(), mInnerBox.XMost());
+    rect.x = leftEdge;
+    rect.width = rightEdge - leftEdge;
+
+    // Should have returned false above
+    NS_ASSERTION(rect.width >= 0, "horz scroll code out of sync");
+  }
+
+  return true;
+}
+
+bool nsTreeBodyFrame::CanAutoScroll(int32_t aRowIndex) {
+  // Check first for partially visible last row.
+  if (aRowIndex == mRowCount - 1) {
+    nscoord y = mInnerBox.y + (aRowIndex - mTopRowIndex) * mRowHeight;
+    if (y < mInnerBox.height && y + mRowHeight > mInnerBox.height) return true;
+  }
+
+  if (aRowIndex > 0 && aRowIndex < mRowCount - 1) return true;
+
+  return false;
+}
+
+// Given a dom event, figure out which row in the tree the mouse is over,
+// if we should drop before/after/on that row or we should auto-scroll.
+// Doesn't query the content about if the drag is allowable, that's done
+// elsewhere.
+//
+// For containers, we break up the vertical space of the row as follows: if in
+// the topmost 25%, the drop is _before_ the row the mouse is over; if in the
+// last 25%, _after_; in the middle 50%, we consider it a drop _on_ the
+// container.
+//
+// For non-containers, if the mouse is in the top 50% of the row, the drop is
+// _before_ and the bottom 50% _after_
+void nsTreeBodyFrame::ComputeDropPosition(WidgetGUIEvent* aEvent, int32_t* aRow,
+                                          int16_t* aOrient,
+                                          int16_t* aScrollLines) {
+  *aOrient = -1;
+  *aScrollLines = 0;
+
+  // Convert the event's point to our coordinates.  We want it in
+  // the coordinates of our inner box's coordinates.
+  nsPoint pt =
+      nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, RelativeTo{this});
+  int32_t xTwips = pt.x - mInnerBox.x;
+  int32_t yTwips = pt.y - mInnerBox.y;
+
+  nsCOMPtr<nsITreeView> view = GetExistingView();
+  *aRow = GetRowAtInternal(xTwips, yTwips);
+  if (*aRow >= 0) {
+    // Compute the top/bottom of the row in question.
+    int32_t yOffset = yTwips - mRowHeight * (*aRow - mTopRowIndex);
+
+    bool isContainer = false;
+    view->IsContainer(*aRow, &isContainer);
+    if (isContainer) {
+      // for a container, use a 25%/50%/25% breakdown
+      if (yOffset < mRowHeight / 4)
+        *aOrient = nsITreeView::DROP_BEFORE;
+      else if (yOffset > mRowHeight - (mRowHeight / 4))
+        *aOrient = nsITreeView::DROP_AFTER;
+      else
+        *aOrient = nsITreeView::DROP_ON;
+    } else {
+      // for a non-container use a 50%/50% breakdown
+      if (yOffset < mRowHeight / 2)
+        *aOrient = nsITreeView::DROP_BEFORE;
+      else
+        *aOrient = nsITreeView::DROP_AFTER;
+    }
+  }
+
+  if (CanAutoScroll(*aRow)) {
+    // Get the max value from the look and feel service.
+    int32_t scrollLinesMax =
+        LookAndFeel::GetInt(LookAndFeel::IntID::TreeScrollLinesMax, 0);
+    scrollLinesMax--;
+    if (scrollLinesMax < 0) scrollLinesMax = 0;
+
+    // Determine if we're w/in a margin of the top/bottom of the tree during a
+    // drag. This will ultimately cause us to scroll, but that's done elsewhere.
+    nscoord height = (3 * mRowHeight) / 4;
+    if (yTwips < height) {
+      // scroll up
+      *aScrollLines =
+          NSToIntRound(-scrollLinesMax * (1 - (float)yTwips / height) - 1);
+    } else if (yTwips > mRect.height - height) {
+      // scroll down
+      *aScrollLines = NSToIntRound(
+          scrollLinesMax * (1 - (float)(mRect.height - yTwips) / height) + 1);
+    }
+  }
+}  // ComputeDropPosition
+
+void nsTreeBodyFrame::OpenCallback(nsITimer* aTimer, void* aClosure) {
+  auto* self = static_cast<nsTreeBodyFrame*>(aClosure);
+  if (!self) {
+    return;
+  }
+
+  aTimer->Cancel();
+  self->mSlots->mTimer = nullptr;
+
+  nsCOMPtr<nsITreeView> view = self->GetExistingView();
+  if (self->mSlots->mDropRow >= 0) {
+    self->mSlots->mArray.AppendElement(self->mSlots->mDropRow);
+    view->ToggleOpenState(self->mSlots->mDropRow);
+  }
+}
+
+void nsTreeBodyFrame::CloseCallback(nsITimer* aTimer, void* aClosure) {
+  auto* self = static_cast<nsTreeBodyFrame*>(aClosure);
+  if (!self) {
+    return;
+  }
+
+  aTimer->Cancel();
+  self->mSlots->mTimer = nullptr;
+
+  nsCOMPtr<nsITreeView> view = self->GetExistingView();
+  auto array = std::move(self->mSlots->mArray);
+  if (!view) {
+    return;
+  }
+  for (auto elem : Reversed(array)) {
+    view->ToggleOpenState(elem);
+  }
+}
+
+void nsTreeBodyFrame::LazyScrollCallback(nsITimer* aTimer, void* aClosure) {
+  nsTreeBodyFrame* self = static_cast<nsTreeBodyFrame*>(aClosure);
+  if (self) {
+    aTimer->Cancel();
+    self->mSlots->mTimer = nullptr;
+
+    if (self->mView) {
+      // Set a new timer to scroll the tree repeatedly.
+      self->CreateTimer(LookAndFeel::IntID::TreeScrollDelay, ScrollCallback,
+                        nsITimer::TYPE_REPEATING_SLACK,
+                        getter_AddRefs(self->mSlots->mTimer),
+                        "nsTreeBodyFrame::ScrollCallback");
+      self->ScrollByLines(self->mSlots->mScrollLines);
+      // ScrollByLines may have deleted |self|.
+    }
+  }
+}
+
+void nsTreeBodyFrame::ScrollCallback(nsITimer* aTimer, void* aClosure) {
+  nsTreeBodyFrame* self = static_cast<nsTreeBodyFrame*>(aClosure);
+  if (self) {
+    // Don't scroll if we are already at the top or bottom of the view.
+    if (self->mView && self->CanAutoScroll(self->mSlots->mDropRow)) {
+      self->ScrollByLines(self->mSlots->mScrollLines);
+    } else {
+      aTimer->Cancel();
+      self->mSlots->mTimer = nullptr;
+    }
+  }
+}
+
+// TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230, bug 1535398)
+MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHODIMP nsTreeBodyFrame::ScrollEvent::Run() {
+  if (mInner) {
+    mInner->FireScrollEvent();
+  }
+  return NS_OK;
+}
+
+void nsTreeBodyFrame::FireScrollEvent() {
+  mScrollEvent.Forget();
+  WidgetGUIEvent event(true, eScroll, nullptr);
+  // scroll events fired at elements don't bubble
+  event.mFlags.mBubbles = false;
+  RefPtr<nsIContent> content = GetContent();
+  RefPtr<nsPresContext> presContext = PresContext();
+  EventDispatcher::Dispatch(content, presContext, &event);
+}
+
+void nsTreeBodyFrame::PostScrollEvent() {
+  if (mScrollEvent.IsPending()) return;
+
+  RefPtr<ScrollEvent> event = new ScrollEvent(this);
+  nsresult rv =
+      mContent->OwnerDoc()->Dispatch(TaskCategory::Other, do_AddRef(event));
+  if (NS_FAILED(rv)) {
+    NS_WARNING("failed to dispatch ScrollEvent");
+  } else {
+    mScrollEvent = std::move(event);
+  }
+}
+
+void nsTreeBodyFrame::ScrollbarActivityStarted() const {
+  if (mScrollbarActivity) {
+    mScrollbarActivity->ActivityStarted();
+  }
+}
+
+void nsTreeBodyFrame::ScrollbarActivityStopped() const {
+  if (mScrollbarActivity) {
+    mScrollbarActivity->ActivityStopped();
+  }
+}
+
+void nsTreeBodyFrame::DetachImageListeners() { mCreatedListeners.Clear(); }
+
+void nsTreeBodyFrame::RemoveTreeImageListener(nsTreeImageListener* aListener) {
+  if (aListener) {
+    mCreatedListeners.Remove(aListener);
+  }
+}
+
+#ifdef ACCESSIBILITY
+static void InitCustomEvent(CustomEvent* aEvent, const nsAString& aType,
+                            nsIWritablePropertyBag2* aDetail) {
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(aEvent->GetParentObject())) {
+    return;
+  }
+
+  JSContext* cx = jsapi.cx();
+  JS::Rooted<JS::Value> detail(cx);
+  if (!ToJSValue(cx, aDetail, &detail)) {
+    jsapi.ClearException();
+    return;
+  }
+
+  aEvent->InitCustomEvent(cx, aType, /* aCanBubble = */ true,
+                          /* aCancelable = */ false, detail);
+}
+
+void nsTreeBodyFrame::FireRowCountChangedEvent(int32_t aIndex, int32_t aCount) {
+  RefPtr<XULTreeElement> tree(GetBaseElement());
+  if (!tree) return;
+
+  RefPtr<Document> doc = tree->OwnerDoc();
+  MOZ_ASSERT(doc);
+
+  RefPtr<Event> event =
+      doc->CreateEvent(u"customevent"_ns, CallerType::System, IgnoreErrors());
+
+  CustomEvent* treeEvent = event->AsCustomEvent();
+  if (!treeEvent) {
+    return;
+  }
+
+  nsCOMPtr<nsIWritablePropertyBag2> propBag(
+      do_CreateInstance("@mozilla.org/hash-property-bag;1"));
+  if (!propBag) {
+    return;
+  }
+
+  // Set 'index' data - the row index rows are changed from.
+  propBag->SetPropertyAsInt32(u"index"_ns, aIndex);
+
+  // Set 'count' data - the number of changed rows.
+  propBag->SetPropertyAsInt32(u"count"_ns, aCount);
+
+  InitCustomEvent(treeEvent, u"TreeRowCountChanged"_ns, propBag);
+
+  event->SetTrusted(true);
+
+  RefPtr<AsyncEventDispatcher> asyncDispatcher =
+      new AsyncEventDispatcher(tree, event);
+  asyncDispatcher->PostDOMEvent();
+}
+
+void nsTreeBodyFrame::FireInvalidateEvent(int32_t aStartRowIdx,
+                                          int32_t aEndRowIdx,
+                                          nsTreeColumn* aStartCol,
+                                          nsTreeColumn* aEndCol) {
+  RefPtr<XULTreeElement> tree(GetBaseElement());
+  if (!tree) return;
+
+  RefPtr<Document> doc = tree->OwnerDoc();
+
+  RefPtr<Event> event =
+      doc->CreateEvent(u"customevent"_ns, CallerType::System, IgnoreErrors());
+
+  CustomEvent* treeEvent = event->AsCustomEvent();
+  if (!treeEvent) {
+    return;
+  }
+
+  nsCOMPtr<nsIWritablePropertyBag2> propBag(
+      do_CreateInstance("@mozilla.org/hash-property-bag;1"));
+  if (!propBag) {
+    return;
+  }
+
+  if (aStartRowIdx != -1 && aEndRowIdx != -1) {
+    // Set 'startrow' data - the start index of invalidated rows.
+    propBag->SetPropertyAsInt32(u"startrow"_ns, aStartRowIdx);
+
+    // Set 'endrow' data - the end index of invalidated rows.
+    propBag->SetPropertyAsInt32(u"endrow"_ns, aEndRowIdx);
+  }
+
+  if (aStartCol && aEndCol) {
+    // Set 'startcolumn' data - the start index of invalidated rows.
+    int32_t startColIdx = aStartCol->GetIndex();
+
+    propBag->SetPropertyAsInt32(u"startcolumn"_ns, startColIdx);
+
+    // Set 'endcolumn' data - the start index of invalidated rows.
+    int32_t endColIdx = aEndCol->GetIndex();
+    propBag->SetPropertyAsInt32(u"endcolumn"_ns, endColIdx);
+  }
+
+  InitCustomEvent(treeEvent, u"TreeInvalidated"_ns, propBag);
+
+  event->SetTrusted(true);
+
+  RefPtr<AsyncEventDispatcher> asyncDispatcher =
+      new AsyncEventDispatcher(tree, event);
+  asyncDispatcher->PostDOMEvent();
+}
+#endif
+
+class nsOverflowChecker : public Runnable {
+ public:
+  explicit nsOverflowChecker(nsTreeBodyFrame* aFrame)
+      : mozilla::Runnable("nsOverflowChecker"), mFrame(aFrame) {}
+  NS_IMETHOD Run() override {
+    if (mFrame.IsAlive()) {
+      nsTreeBodyFrame* tree = static_cast<nsTreeBodyFrame*>(mFrame.GetFrame());
+      nsTreeBodyFrame::ScrollParts parts = tree->GetScrollParts();
+      tree->CheckOverflow(parts);
+    }
+    return NS_OK;
+  }
+
+ private:
+  WeakFrame mFrame;
+};
+
+bool nsTreeBodyFrame::FullScrollbarsUpdate(bool aNeedsFullInvalidation) {
+  ScrollParts parts = GetScrollParts();
+  AutoWeakFrame weakFrame(this);
+  AutoWeakFrame weakColumnsFrame(parts.mColumnsFrame);
+  UpdateScrollbars(parts);
+  NS_ENSURE_TRUE(weakFrame.IsAlive(), false);
+  if (aNeedsFullInvalidation) {
+    Invalidate();
+  }
+  InvalidateScrollbars(parts, weakColumnsFrame);
+  NS_ENSURE_TRUE(weakFrame.IsAlive(), false);
+
+  // Overflow checking dispatches synchronous events, which can cause infinite
+  // recursion during reflow. Do the first overflow check synchronously, but
+  // force any nested checks to round-trip through the event loop. See bug
+  // 905909.
+  RefPtr<nsOverflowChecker> checker = new nsOverflowChecker(this);
+  if (!mCheckingOverflow) {
+    nsContentUtils::AddScriptRunner(checker);
+  } else {
+    mContent->OwnerDoc()->Dispatch(TaskCategory::Other, checker.forget());
+  }
+  return weakFrame.IsAlive();
+}
+
+void nsTreeBodyFrame::OnImageIsAnimated(imgIRequest* aRequest) {
+  nsLayoutUtils::RegisterImageRequest(PresContext(), aRequest, nullptr);
+}
diff --git a/layout/xul/tree/nsTreeBodyFrame.h b/layout/xul/tree/nsTreeBodyFrame.h
new file mode 100644
index 0000000000..35fa7eb1f0
--- /dev/null
+++ b/layout/xul/tree/nsTreeBodyFrame.h
@@ -0,0 +1,607 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeBodyFrame_h
+#define nsTreeBodyFrame_h
+
+#include "mozilla/AtomArray.h"
+#include "mozilla/Attributes.h"
+
+#include "nsITreeView.h"
+#include "nsIScrollbarMediator.h"
+#include "nsITimer.h"
+#include "nsIReflowCallback.h"
+#include "nsTArray.h"
+#include "nsTreeStyleCache.h"
+#include "nsTreeColumns.h"
+#include "nsTHashMap.h"
+#include "nsTHashSet.h"
+#include "imgIRequest.h"
+#include "imgINotificationObserver.h"
+#include "nsScrollbarFrame.h"
+#include "nsThreadUtils.h"
+#include "SimpleXULLeafFrame.h"
+#include "mozilla/LookAndFeel.h"
+
+class nsFontMetrics;
+class nsOverflowChecker;
+class nsTreeImageListener;
+
+namespace mozilla {
+class PresShell;
+namespace layout {
+class ScrollbarActivity;
+}  // namespace layout
+}  // namespace mozilla
+
+// An entry in the tree's image cache
+struct nsTreeImageCacheEntry {
+  nsTreeImageCacheEntry() = default;
+  nsTreeImageCacheEntry(imgIRequest* aRequest,
+                        imgINotificationObserver* aListener)
+      : request(aRequest), listener(aListener) {}
+
+  nsCOMPtr<imgIRequest> request;
+  nsCOMPtr<imgINotificationObserver> listener;
+};
+
+// The actual frame that paints the cells and rows.
+class nsTreeBodyFrame final : public mozilla::SimpleXULLeafFrame,
+                              public nsIScrollbarMediator,
+                              public nsIReflowCallback {
+  typedef mozilla::layout::ScrollbarActivity ScrollbarActivity;
+  typedef mozilla::image::ImgDrawResult ImgDrawResult;
+
+ public:
+  explicit nsTreeBodyFrame(ComputedStyle* aStyle, nsPresContext* aPresContext);
+  ~nsTreeBodyFrame();
+
+  nscoord GetIntrinsicBSize() override;
+
+  NS_DECL_QUERYFRAME
+  NS_DECL_FRAMEARENA_HELPERS(nsTreeBodyFrame)
+
+  // Callback handler methods for refresh driver based animations.
+  // Calls to these functions are forwarded from nsTreeImageListener. These
+  // mirror how nsImageFrame works.
+  void OnImageIsAnimated(imgIRequest* aRequest);
+
+  // non-virtual signatures like nsITreeBodyFrame
+  already_AddRefed<nsTreeColumns> Columns() const {
+    RefPtr<nsTreeColumns> cols = mColumns;
+    return cols.forget();
+  }
+  already_AddRefed<nsITreeView> GetExistingView() const {
+    nsCOMPtr<nsITreeView> view = mView;
+    return view.forget();
+  }
+  already_AddRefed<nsITreeSelection> GetSelection() const;
+  nsresult GetView(nsITreeView** aView);
+  nsresult SetView(nsITreeView* aView);
+  bool GetFocused() const { return mFocused; }
+  nsresult SetFocused(bool aFocused);
+  nsresult GetTreeBody(mozilla::dom::Element** aElement);
+  int32_t RowHeight() const;
+  int32_t RowWidth();
+  int32_t GetHorizontalPosition() const;
+  mozilla::Maybe<mozilla::CSSIntRegion> GetSelectionRegion();
+  int32_t FirstVisibleRow() const { return mTopRowIndex; }
+  int32_t LastVisibleRow() const { return mTopRowIndex + mPageLength; }
+  int32_t PageLength() const { return mPageLength; }
+  nsresult EnsureRowIsVisible(int32_t aRow);
+  nsresult EnsureCellIsVisible(int32_t aRow, nsTreeColumn* aCol);
+  void ScrollToRow(int32_t aRow);
+  void ScrollByLines(int32_t aNumLines);
+  void ScrollByPages(int32_t aNumPages);
+  nsresult Invalidate();
+  nsresult InvalidateColumn(nsTreeColumn* aCol);
+  nsresult InvalidateRow(int32_t aRow);
+  nsresult InvalidateCell(int32_t aRow, nsTreeColumn* aCol);
+  nsresult InvalidateRange(int32_t aStart, int32_t aEnd);
+  int32_t GetRowAt(int32_t aX, int32_t aY);
+  nsresult GetCellAt(int32_t aX, int32_t aY, int32_t* aRow, nsTreeColumn** aCol,
+                     nsACString& aChildElt);
+  nsresult GetCoordsForCellItem(int32_t aRow, nsTreeColumn* aCol,
+                                const nsACString& aElt, int32_t* aX,
+                                int32_t* aY, int32_t* aWidth, int32_t* aHeight);
+  nsresult IsCellCropped(int32_t aRow, nsTreeColumn* aCol, bool* aResult);
+  nsresult RowCountChanged(int32_t aIndex, int32_t aCount);
+  nsresult BeginUpdateBatch();
+  nsresult EndUpdateBatch();
+  nsresult ClearStyleAndImageCaches();
+  void RemoveImageCacheEntry(int32_t aRowIndex, nsTreeColumn* aCol);
+
+  void CancelImageRequests();
+
+  void ManageReflowCallback();
+
+  void DidReflow(nsPresContext*, const ReflowInput*) override;
+
+  // nsIReflowCallback
+  bool ReflowFinished() override;
+  void ReflowCallbackCanceled() override;
+
+  // nsIScrollbarMediator
+  void ScrollByPage(nsScrollbarFrame* aScrollbar, int32_t aDirection,
+                    mozilla::ScrollSnapFlags aSnapFlags =
+                        mozilla::ScrollSnapFlags::Disabled) override;
+  void ScrollByWhole(nsScrollbarFrame* aScrollbar, int32_t aDirection,
+                     mozilla::ScrollSnapFlags aSnapFlags =
+                         mozilla::ScrollSnapFlags::Disabled) override;
+  void ScrollByLine(nsScrollbarFrame* aScrollbar, int32_t aDirection,
+                    mozilla::ScrollSnapFlags aSnapFlags =
+                        mozilla::ScrollSnapFlags::Disabled) override;
+  void ScrollByUnit(nsScrollbarFrame* aScrollbar, mozilla::ScrollMode aMode,
+                    int32_t aDirection, mozilla::ScrollUnit aUnit,
+                    mozilla::ScrollSnapFlags aSnapFlags =
+                        mozilla::ScrollSnapFlags::Disabled) override;
+  void RepeatButtonScroll(nsScrollbarFrame* aScrollbar) override;
+  void ThumbMoved(nsScrollbarFrame* aScrollbar, nscoord aOldPos,
+                  nscoord aNewPos) override;
+  void ScrollbarReleased(nsScrollbarFrame* aScrollbar) override {}
+  void VisibilityChanged(bool aVisible) override { Invalidate(); }
+  nsScrollbarFrame* GetScrollbarBox(bool aVertical) override {
+    ScrollParts parts = GetScrollParts();
+    return aVertical ? parts.mVScrollbar : parts.mHScrollbar;
+  }
+  void ScrollbarActivityStarted() const override;
+  void ScrollbarActivityStopped() const override;
+  bool IsScrollbarOnRight() const override {
+    return StyleVisibility()->mDirection == mozilla::StyleDirection::Ltr;
+  }
+  bool ShouldSuppressScrollbarRepaints() const override { return false; }
+
+  // Overridden from nsIFrame to cache our pres context.
+  void Init(nsIContent* aContent, nsContainerFrame* aParent,
+            nsIFrame* aPrevInFlow) override;
+  void Destroy(DestroyContext&) override;
+
+  mozilla::Maybe<Cursor> GetCursor(const nsPoint&) override;
+
+  nsresult HandleEvent(nsPresContext* aPresContext,
+                       mozilla::WidgetGUIEvent* aEvent,
+                       nsEventStatus* aEventStatus) override;
+
+  void BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                        const nsDisplayListSet& aLists) override;
+
+  void DidSetComputedStyle(ComputedStyle* aOldComputedStyle) override;
+
+  friend nsIFrame* NS_NewTreeBodyFrame(mozilla::PresShell* aPresShell);
+  friend class nsTreeColumn;
+
+  struct ScrollParts {
+    nsScrollbarFrame* mVScrollbar;
+    RefPtr<mozilla::dom::Element> mVScrollbarContent;
+    nsScrollbarFrame* mHScrollbar;
+    RefPtr<mozilla::dom::Element> mHScrollbarContent;
+    nsIFrame* mColumnsFrame;
+    nsIScrollableFrame* mColumnsScrollFrame;
+  };
+
+  ImgDrawResult PaintTreeBody(gfxContext& aRenderingContext,
+                              const nsRect& aDirtyRect, nsPoint aPt,
+                              nsDisplayListBuilder* aBuilder);
+
+  // Get the base element, <tree>
+  mozilla::dom::XULTreeElement* GetBaseElement();
+
+  bool GetVerticalOverflow() const { return mVerticalOverflow; }
+  bool GetHorizontalOverflow() const { return mHorizontalOverflow; }
+
+  // This returns the property array where atoms are stored for style during
+  // draw, whether the row currently being drawn is selected, hovered, etc.
+  const mozilla::AtomArray& GetPropertyArrayForCurrentDrawingItem() {
+    return mScratchArray;
+  }
+
+ protected:
+  friend class nsOverflowChecker;
+
+  // This method paints a specific column background of the tree.
+  ImgDrawResult PaintColumn(nsTreeColumn* aColumn, const nsRect& aColumnRect,
+                            nsPresContext* aPresContext,
+                            gfxContext& aRenderingContext,
+                            const nsRect& aDirtyRect);
+
+  // This method paints a single row in the tree.
+  ImgDrawResult PaintRow(int32_t aRowIndex, const nsRect& aRowRect,
+                         nsPresContext* aPresContext,
+                         gfxContext& aRenderingContext,
+                         const nsRect& aDirtyRect, nsPoint aPt,
+                         nsDisplayListBuilder* aBuilder);
+
+  // This method paints a single separator in the tree.
+  ImgDrawResult PaintSeparator(int32_t aRowIndex, const nsRect& aSeparatorRect,
+                               nsPresContext* aPresContext,
+                               gfxContext& aRenderingContext,
+                               const nsRect& aDirtyRect);
+
+  // This method paints a specific cell in a given row of the tree.
+  ImgDrawResult PaintCell(int32_t aRowIndex, nsTreeColumn* aColumn,
+                          const nsRect& aCellRect, nsPresContext* aPresContext,
+                          gfxContext& aRenderingContext,
+                          const nsRect& aDirtyRect, nscoord& aCurrX,
+                          nsPoint aPt, nsDisplayListBuilder* aBuilder);
+
+  // This method paints the twisty inside a cell in the primary column of an
+  // tree.
+  ImgDrawResult PaintTwisty(int32_t aRowIndex, nsTreeColumn* aColumn,
+                            const nsRect& aTwistyRect,
+                            nsPresContext* aPresContext,
+                            gfxContext& aRenderingContext,
+                            const nsRect& aDirtyRect, nscoord& aRemainingWidth,
+                            nscoord& aCurrX);
+
+  // This method paints the image inside the cell of an tree.
+  ImgDrawResult PaintImage(int32_t aRowIndex, nsTreeColumn* aColumn,
+                           const nsRect& aImageRect,
+                           nsPresContext* aPresContext,
+                           gfxContext& aRenderingContext,
+                           const nsRect& aDirtyRect, nscoord& aRemainingWidth,
+                           nscoord& aCurrX, nsDisplayListBuilder* aBuilder);
+
+  // This method paints the text string inside a particular cell of the tree.
+  ImgDrawResult PaintText(int32_t aRowIndex, nsTreeColumn* aColumn,
+                          const nsRect& aTextRect, nsPresContext* aPresContext,
+                          gfxContext& aRenderingContext,
+                          const nsRect& aDirtyRect, nscoord& aCurrX);
+
+  // This method paints the checkbox inside a particular cell of the tree.
+  ImgDrawResult PaintCheckbox(int32_t aRowIndex, nsTreeColumn* aColumn,
+                              const nsRect& aCheckboxRect,
+                              nsPresContext* aPresContext,
+                              gfxContext& aRenderingContext,
+                              const nsRect& aDirtyRect);
+
+  // This method paints a drop feedback of the tree.
+  ImgDrawResult PaintDropFeedback(const nsRect& aDropFeedbackRect,
+                                  nsPresContext* aPresContext,
+                                  gfxContext& aRenderingContext,
+                                  const nsRect& aDirtyRect, nsPoint aPt);
+
+  // This method is called with a specific ComputedStyle and rect to
+  // paint the background rect as if it were a full-blown frame.
+  ImgDrawResult PaintBackgroundLayer(ComputedStyle* aComputedStyle,
+                                     nsPresContext* aPresContext,
+                                     gfxContext& aRenderingContext,
+                                     const nsRect& aRect,
+                                     const nsRect& aDirtyRect);
+
+  // An internal hit test.  aX and aY are expected to be in twips in the
+  // coordinate system of this frame.
+  int32_t GetRowAtInternal(nscoord aX, nscoord aY);
+
+  // Check for bidi characters in the text, and if there are any, ensure
+  // that the prescontext is in bidi mode.
+  void CheckTextForBidi(nsAutoString& aText);
+
+  void AdjustForCellText(nsAutoString& aText, int32_t aRowIndex,
+                         nsTreeColumn* aColumn, gfxContext& aRenderingContext,
+                         nsFontMetrics& aFontMetrics, nsRect& aTextRect);
+
+  // A helper used when hit testing.
+  nsCSSAnonBoxPseudoStaticAtom* GetItemWithinCellAt(nscoord aX,
+                                                    const nsRect& aCellRect,
+                                                    int32_t aRowIndex,
+                                                    nsTreeColumn* aColumn);
+
+  // An internal hit test.  aX and aY are expected to be in twips in the
+  // coordinate system of this frame.
+  void GetCellAt(nscoord aX, nscoord aY, int32_t* aRow, nsTreeColumn** aCol,
+                 nsCSSAnonBoxPseudoStaticAtom** aChildElt);
+
+  // Retrieve the area for the twisty for a cell.
+  nsITheme* GetTwistyRect(int32_t aRowIndex, nsTreeColumn* aColumn,
+                          nsRect& aImageRect, nsRect& aTwistyRect,
+                          nsPresContext* aPresContext,
+                          ComputedStyle* aTwistyContext);
+
+  // Fetch an image from the image cache.
+  nsresult GetImage(int32_t aRowIndex, nsTreeColumn* aCol, bool aUseContext,
+                    ComputedStyle* aComputedStyle, imgIContainer** aResult);
+
+  // Returns the size of a given image.   This size *includes* border and
+  // padding.  It does not include margins.
+  nsRect GetImageSize(int32_t aRowIndex, nsTreeColumn* aCol, bool aUseContext,
+                      ComputedStyle* aComputedStyle);
+
+  // Returns the destination size of the image, not including borders and
+  // padding.
+  nsSize GetImageDestSize(ComputedStyle*, imgIContainer*);
+
+  // Returns the source rectangle of the image to be displayed.
+  nsRect GetImageSourceRect(ComputedStyle*, imgIContainer*);
+
+  // Returns the height of rows in the tree.
+  int32_t GetRowHeight();
+
+  // Returns our indentation width.
+  int32_t GetIndentation();
+
+  // Calculates our width/height once border and padding have been removed.
+  void CalcInnerBox();
+
+  // Calculate the total width of our scrollable portion
+  nscoord CalcHorzWidth(const ScrollParts& aParts);
+
+  // Looks up a ComputedStyle in the style cache.  On a cache miss we resolve
+  // the pseudo-styles passed in and place them into the cache.
+  ComputedStyle* GetPseudoComputedStyle(
+      nsCSSAnonBoxPseudoStaticAtom* aPseudoElement);
+
+  // Retrieves the scrollbars and scrollview relevant to this treebody. We
+  // traverse the frame tree under our base element, in frame order, looking
+  // for the first relevant vertical scrollbar, horizontal scrollbar, and
+  // scrollable frame (with associated content and scrollable view). These
+  // are all volatile and should not be retained.
+  ScrollParts GetScrollParts();
+
+  // Update the curpos of the scrollbar.
+  void UpdateScrollbars(const ScrollParts& aParts);
+
+  // Update the maxpos of the scrollbar.
+  void InvalidateScrollbars(const ScrollParts& aParts,
+                            AutoWeakFrame& aWeakColumnsFrame);
+
+  // Check overflow and generate events.
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void CheckOverflow(const ScrollParts& aParts);
+
+  // Calls UpdateScrollbars, Invalidate aNeedsFullInvalidation if true,
+  // InvalidateScrollbars and finally CheckOverflow.
+  // returns true if the frame is still alive after the method call.
+  bool FullScrollbarsUpdate(bool aNeedsFullInvalidation);
+
+  // Use to auto-fill some of the common properties without the view having to
+  // do it. Examples include container, open, selected, and focus.
+  void PrefillPropertyArray(int32_t aRowIndex, nsTreeColumn* aCol);
+
+  // Our internal scroll method, used by all the public scroll methods.
+  nsresult ScrollInternal(const ScrollParts& aParts, int32_t aRow);
+  nsresult ScrollToRowInternal(const ScrollParts& aParts, int32_t aRow);
+  nsresult ScrollHorzInternal(const ScrollParts& aParts, int32_t aPosition);
+  nsresult EnsureRowIsVisibleInternal(const ScrollParts& aParts, int32_t aRow);
+
+  // Convert client pixels into appunits in our coordinate space.
+  nsPoint AdjustClientCoordsToBoxCoordSpace(int32_t aX, int32_t aY);
+
+  void EnsureView();
+
+  nsresult GetCellWidth(int32_t aRow, nsTreeColumn* aCol,
+                        gfxContext* aRenderingContext, nscoord& aDesiredSize,
+                        nscoord& aCurrentSize);
+
+  // Translate the given rect horizontally from tree coordinates into the
+  // coordinate system of our nsTreeBodyFrame.  If clip is true, then clip the
+  // rect to its intersection with mInnerBox in the horizontal direction.
+  // Return whether the result has a nonempty intersection with mInnerBox
+  // after projecting both onto the horizontal coordinate axis.
+  bool OffsetForHorzScroll(nsRect& rect, bool clip);
+
+  bool CanAutoScroll(int32_t aRowIndex);
+
+  // Calc the row and above/below/on status given where the mouse currently is
+  // hovering. Also calc if we're in the region in which we want to auto-scroll
+  // the tree. A positive value of |aScrollLines| means scroll down, a negative
+  // value means scroll up, a zero value means that we aren't in drag scroll
+  // region.
+  void ComputeDropPosition(mozilla::WidgetGUIEvent* aEvent, int32_t* aRow,
+                           int16_t* aOrient, int16_t* aScrollLines);
+
+  void InvalidateDropFeedback(int32_t aRow, int16_t aOrientation) {
+    InvalidateRow(aRow);
+    if (aOrientation != nsITreeView::DROP_ON)
+      InvalidateRow(aRow + aOrientation);
+  }
+
+ public:
+  /**
+   * Remove an nsITreeImageListener from being tracked by this frame. Only tree
+   * image listeners that are created by this frame are tracked.
+   *
+   * @param aListener A pointer to an nsTreeImageListener to no longer
+   *        track.
+   */
+  void RemoveTreeImageListener(nsTreeImageListener* aListener);
+
+ protected:
+  // Create a new timer. This method is used to delay various actions like
+  // opening/closing folders or tree scrolling.
+  // aID is type of the action, aFunc is the function to be called when
+  // the timer fires and aType is type of timer - one shot or repeating.
+  nsresult CreateTimer(const mozilla::LookAndFeel::IntID aID,
+                       nsTimerCallbackFunc aFunc, int32_t aType,
+                       nsITimer** aTimer, const char* aName);
+
+  static void OpenCallback(nsITimer* aTimer, void* aClosure);
+
+  static void CloseCallback(nsITimer* aTimer, void* aClosure);
+
+  static void LazyScrollCallback(nsITimer* aTimer, void* aClosure);
+
+  static void ScrollCallback(nsITimer* aTimer, void* aClosure);
+
+  class ScrollEvent : public mozilla::Runnable {
+   public:
+    NS_DECL_NSIRUNNABLE
+    explicit ScrollEvent(nsTreeBodyFrame* aInner)
+        : mozilla::Runnable("nsTreeBodyFrame::ScrollEvent"), mInner(aInner) {}
+    void Revoke() { mInner = nullptr; }
+
+   private:
+    nsTreeBodyFrame* mInner;
+  };
+
+  void PostScrollEvent();
+  MOZ_CAN_RUN_SCRIPT void FireScrollEvent();
+
+  /**
+   * Clear the pointer to this frame for all nsTreeImageListeners that were
+   * created by this frame.
+   */
+  void DetachImageListeners();
+
+#ifdef ACCESSIBILITY
+  /**
+   * Fires 'treeRowCountChanged' event asynchronously. The event is a
+   * CustomEvent that is used to expose the following information structures
+   * via a property bag.
+   *
+   * @param aIndex  the row index rows are added/removed from
+   * @param aCount  the number of added/removed rows (the sign points to
+   *                an operation, plus - addition, minus - removing)
+   */
+  void FireRowCountChangedEvent(int32_t aIndex, int32_t aCount);
+
+  /**
+   * Fires 'treeInvalidated' event asynchronously. The event is a CustomEvent
+   * that is used to expose the information structures described by method
+   * arguments via a property bag.
+   *
+   * @param aStartRow  the start index of invalidated rows, -1 means that
+   *                   columns have been invalidated only
+   * @param aEndRow    the end index of invalidated rows, -1 means that columns
+   *                   have been invalidated only
+   * @param aStartCol  the start invalidated column, nullptr means that only
+   *                   rows have been invalidated
+   * @param aEndCol    the end invalidated column, nullptr means that rows have
+   *                   been invalidated only
+   */
+  void FireInvalidateEvent(int32_t aStartRow, int32_t aEndRow,
+                           nsTreeColumn* aStartCol, nsTreeColumn* aEndCol);
+#endif
+
+ protected:  // Data Members
+  class Slots {
+   public:
+    Slots() = default;
+
+    ~Slots() {
+      if (mTimer) {
+        mTimer->Cancel();
+      }
+    }
+
+    friend class nsTreeBodyFrame;
+
+   protected:
+    // If the drop is actually allowed here or not.
+    bool mDropAllowed = false;
+
+    // True while dragging over the tree.
+    bool mIsDragging = false;
+
+    // The row the mouse is hovering over during a drop.
+    int32_t mDropRow = -1;
+
+    // Where we want to draw feedback (above/on this row/below) if allowed.
+    int16_t mDropOrient = -1;
+
+    // Number of lines to be scrolled.
+    int16_t mScrollLines = 0;
+
+    // The drag action that was received for this slot
+    uint32_t mDragAction = 0;
+
+    // Timer for opening/closing spring loaded folders or scrolling the tree.
+    nsCOMPtr<nsITimer> mTimer;
+
+    // An array used to keep track of all spring loaded folders.
+    nsTArray<int32_t> mArray;
+  };
+
+  mozilla::UniquePtr<Slots> mSlots;
+
+  nsRevocableEventPtr<ScrollEvent> mScrollEvent;
+
+  RefPtr<ScrollbarActivity> mScrollbarActivity;
+
+  // The <tree> element containing this treebody.
+  RefPtr<mozilla::dom::XULTreeElement> mTree;
+
+  // Cached column information.
+  RefPtr<nsTreeColumns> mColumns;
+
+  // The current view for this tree widget.  We get all of our row and cell data
+  // from the view.
+  nsCOMPtr<nsITreeView> mView;
+
+  // A cache of all the ComputedStyles we have seen for rows and cells of the
+  // tree.  This is a mapping from a list of atoms to a corresponding
+  // ComputedStyle.  This cache stores every combination that occurs in the
+  // tree, so for n distinct properties, this cache could have 2 to the n
+  // entries (the power set of all row properties).
+  nsTreeStyleCache mStyleCache;
+
+  // A hashtable that maps from URLs to image request/listener pairs.  The URL
+  // is provided by the view or by the ComputedStyle. The ComputedStyle
+  // represents a resolved :-moz-tree-cell-image (or twisty) pseudo-element.
+  // It maps directly to an imgIRequest.
+  nsTHashMap<nsStringHashKey, nsTreeImageCacheEntry> mImageCache;
+
+  // A scratch array used when looking up cached ComputedStyles.
+  mozilla::AtomArray mScratchArray;
+
+  // The index of the first visible row and the # of rows visible onscreen.
+  // The tree only examines onscreen rows, starting from
+  // this index and going up to index+pageLength.
+  int32_t mTopRowIndex;
+  int32_t mPageLength;
+
+  // The horizontal scroll position
+  nscoord mHorzPosition;
+
+  // The original desired horizontal width before changing it and posting a
+  // reflow callback. In some cases, the desired horizontal width can first be
+  // different from the current desired horizontal width, only to return to
+  // the same value later during the same reflow. In this case, we can cancel
+  // the posted reflow callback and prevent an unnecessary reflow.
+  nscoord mOriginalHorzWidth;
+  // Our desired horizontal width (the width for which we actually have tree
+  // columns).
+  nscoord mHorzWidth;
+  // The amount by which to adjust the width of the last cell.
+  // This depends on whether or not the columnpicker and scrollbars are present.
+  nscoord mAdjustWidth;
+
+  // Our last reflowed rect, used for invalidation, see ManageReflowCallback().
+  Maybe<nsRect> mLastReflowRect;
+
+  // Cached heights and indent info.
+  nsRect mInnerBox;  // 4-byte aligned
+  int32_t mRowHeight;
+  int32_t mIndentation;
+
+  int32_t mUpdateBatchNest;
+
+  // Cached row count.
+  int32_t mRowCount;
+
+  // The row the mouse is hovering over.
+  int32_t mMouseOverRow;
+
+  // Whether or not we're currently focused.
+  bool mFocused;
+
+  // Do we have a fixed number of onscreen rows?
+  bool mHasFixedRowCount;
+
+  bool mVerticalOverflow;
+  bool mHorizontalOverflow;
+
+  bool mReflowCallbackPosted;
+
+  // Set while we flush layout to take account of effects of
+  // overflow/underflow event handlers
+  bool mCheckingOverflow;
+
+  // Hash set to keep track of which listeners we created and thus
+  // have pointers to us.
+  nsTHashSet<nsTreeImageListener*> mCreatedListeners;
+
+};  // class nsTreeBodyFrame
+
+#endif
diff --git a/layout/xul/tree/nsTreeColumns.cpp b/layout/xul/tree/nsTreeColumns.cpp
new file mode 100644
index 0000000000..62a8299c6f
--- /dev/null
+++ b/layout/xul/tree/nsTreeColumns.cpp
@@ -0,0 +1,462 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsNameSpaceManager.h"
+#include "nsGkAtoms.h"
+#include "nsTreeColumns.h"
+#include "nsTreeUtils.h"
+#include "mozilla/ComputedStyle.h"
+#include "nsContentUtils.h"
+#include "nsTreeBodyFrame.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/CSSOrderAwareFrameIterator.h"
+#include "mozilla/dom/TreeColumnBinding.h"
+#include "mozilla/dom/TreeColumnsBinding.h"
+#include "mozilla/dom/XULTreeElement.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+// Column class that caches all the info about our column.
+nsTreeColumn::nsTreeColumn(nsTreeColumns* aColumns, dom::Element* aElement)
+    : mContent(aElement), mColumns(aColumns), mIndex(0), mPrevious(nullptr) {
+  NS_ASSERTION(aElement && aElement->NodeInfo()->Equals(nsGkAtoms::treecol,
+                                                        kNameSpaceID_XUL),
+               "nsTreeColumn's content must be a <xul:treecol>");
+
+  Invalidate(IgnoreErrors());
+}
+
+nsTreeColumn::~nsTreeColumn() {
+  if (mNext) {
+    mNext->SetPrevious(nullptr);
+  }
+}
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(nsTreeColumn)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsTreeColumn)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mContent)
+  if (tmp->mNext) {
+    tmp->mNext->SetPrevious(nullptr);
+    NS_IMPL_CYCLE_COLLECTION_UNLINK(mNext)
+  }
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsTreeColumn)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mContent)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mNext)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsTreeColumn)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsTreeColumn)
+
+// QueryInterface implementation for nsTreeColumn
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsTreeColumn)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+  NS_INTERFACE_MAP_ENTRY_CONCRETE(nsTreeColumn)
+NS_INTERFACE_MAP_END
+
+nsIFrame* nsTreeColumn::GetFrame() { return mContent->GetPrimaryFrame(); }
+
+bool nsTreeColumn::IsLastVisible(nsTreeBodyFrame* aBodyFrame) {
+  NS_ASSERTION(GetFrame(), "should have checked for this already");
+
+  // cyclers are fixed width, don't adjust them
+  if (IsCycler()) return false;
+
+  // we're certainly not the last visible if we're not visible
+  if (GetFrame()->GetRect().width == 0) return false;
+
+  // try to find a visible successor
+  for (nsTreeColumn* next = GetNext(); next; next = next->GetNext()) {
+    nsIFrame* frame = next->GetFrame();
+    if (frame && frame->GetRect().width > 0) return false;
+  }
+  return true;
+}
+
+nsresult nsTreeColumn::GetRect(nsTreeBodyFrame* aBodyFrame, nscoord aY,
+                               nscoord aHeight, nsRect* aResult) {
+  nsIFrame* frame = GetFrame();
+  if (!frame) {
+    *aResult = nsRect();
+    return NS_ERROR_FAILURE;
+  }
+
+  const bool isRTL =
+      aBodyFrame->StyleVisibility()->mDirection == StyleDirection::Rtl;
+  *aResult = frame->GetRect();
+  if (frame->StyleVisibility()->IsCollapse()) {
+    aResult->SizeTo(nsSize());
+  }
+  aResult->y = aY;
+  aResult->height = aHeight;
+  if (isRTL)
+    aResult->x += aBodyFrame->mAdjustWidth;
+  else if (IsLastVisible(aBodyFrame))
+    aResult->width += aBodyFrame->mAdjustWidth;
+  return NS_OK;
+}
+
+nsresult nsTreeColumn::GetXInTwips(nsTreeBodyFrame* aBodyFrame,
+                                   nscoord* aResult) {
+  nsIFrame* frame = GetFrame();
+  if (!frame) {
+    *aResult = 0;
+    return NS_ERROR_FAILURE;
+  }
+  *aResult = frame->GetRect().x;
+  return NS_OK;
+}
+
+nsresult nsTreeColumn::GetWidthInTwips(nsTreeBodyFrame* aBodyFrame,
+                                       nscoord* aResult) {
+  nsIFrame* frame = GetFrame();
+  if (!frame) {
+    *aResult = 0;
+    return NS_ERROR_FAILURE;
+  }
+  *aResult = frame->GetRect().width;
+  if (IsLastVisible(aBodyFrame)) *aResult += aBodyFrame->mAdjustWidth;
+  return NS_OK;
+}
+
+void nsTreeColumn::GetId(nsAString& aId) const { aId = GetId(); }
+
+void nsTreeColumn::Invalidate(ErrorResult& aRv) {
+  nsIFrame* frame = GetFrame();
+  if (NS_WARN_IF(!frame)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return;
+  }
+
+  // Fetch the Id.
+  mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::id, mId);
+
+  // If we have an Id, cache the Id as an atom.
+  if (!mId.IsEmpty()) {
+    mAtom = NS_Atomize(mId);
+  }
+
+  // Cache our index.
+  nsTreeUtils::GetColumnIndex(mContent, &mIndex);
+
+  const nsStyleVisibility* vis = frame->StyleVisibility();
+
+  // Cache our text alignment policy.
+  const nsStyleText* textStyle = frame->StyleText();
+
+  mTextAlignment = textStyle->mTextAlign;
+  // START or END alignment sometimes means RIGHT
+  if ((mTextAlignment == StyleTextAlign::Start &&
+       vis->mDirection == StyleDirection::Rtl) ||
+      (mTextAlignment == StyleTextAlign::End &&
+       vis->mDirection == StyleDirection::Ltr)) {
+    mTextAlignment = StyleTextAlign::Right;
+  } else if (mTextAlignment == StyleTextAlign::Start ||
+             mTextAlignment == StyleTextAlign::End) {
+    mTextAlignment = StyleTextAlign::Left;
+  }
+
+  // Figure out if we're the primary column (that has to have indentation
+  // and twisties drawn.
+  mIsPrimary = mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::primary,
+                                     nsGkAtoms::_true, eCaseMatters);
+
+  // Figure out if we're a cycling column (one that doesn't cause a selection
+  // to happen).
+  mIsCycler = mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::cycler,
+                                    nsGkAtoms::_true, eCaseMatters);
+
+  mIsEditable = mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::editable,
+                                      nsGkAtoms::_true, eCaseMatters);
+
+  mOverflow = mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::overflow,
+                                    nsGkAtoms::_true, eCaseMatters);
+
+  // Figure out our column type. Default type is text.
+  mType = TreeColumn_Binding::TYPE_TEXT;
+  static Element::AttrValuesArray typestrings[] = {nsGkAtoms::checkbox,
+                                                   nullptr};
+  switch (mContent->FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::type,
+                                    typestrings, eCaseMatters)) {
+    case 0:
+      mType = TreeColumn_Binding::TYPE_CHECKBOX;
+      break;
+  }
+
+  // Fetch the crop style.
+  mCropStyle = 0;
+  static Element::AttrValuesArray cropstrings[] = {
+      nsGkAtoms::center, nsGkAtoms::left, nsGkAtoms::start, nullptr};
+  switch (mContent->FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::crop,
+                                    cropstrings, eCaseMatters)) {
+    case 0:
+      mCropStyle = 1;
+      break;
+    case 1:
+    case 2:
+      mCropStyle = 2;
+      break;
+  }
+}
+
+nsIContent* nsTreeColumn::GetParentObject() const { return mContent; }
+
+/* virtual */
+JSObject* nsTreeColumn::WrapObject(JSContext* aCx,
+                                   JS::Handle<JSObject*> aGivenProto) {
+  return dom::TreeColumn_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+Element* nsTreeColumn::Element() { return mContent; }
+
+int32_t nsTreeColumn::GetX(mozilla::ErrorResult& aRv) {
+  nsIFrame* frame = GetFrame();
+  if (NS_WARN_IF(!frame)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return 0;
+  }
+
+  return nsPresContext::AppUnitsToIntCSSPixels(frame->GetRect().x);
+}
+
+int32_t nsTreeColumn::GetWidth(mozilla::ErrorResult& aRv) {
+  nsIFrame* frame = GetFrame();
+  if (NS_WARN_IF(!frame)) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return 0;
+  }
+
+  return nsPresContext::AppUnitsToIntCSSPixels(frame->GetRect().width);
+}
+
+already_AddRefed<nsTreeColumn> nsTreeColumn::GetPreviousColumn() {
+  return do_AddRef(mPrevious);
+}
+
+nsTreeColumns::nsTreeColumns(nsTreeBodyFrame* aTree) : mTree(aTree) {}
+
+nsTreeColumns::~nsTreeColumns() { nsTreeColumns::InvalidateColumns(); }
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_0(nsTreeColumns)
+
+// QueryInterface implementation for nsTreeColumns
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsTreeColumns)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsTreeColumns)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsTreeColumns)
+
+nsIContent* nsTreeColumns::GetParentObject() const {
+  return mTree ? mTree->GetBaseElement() : nullptr;
+}
+
+/* virtual */
+JSObject* nsTreeColumns::WrapObject(JSContext* aCx,
+                                    JS::Handle<JSObject*> aGivenProto) {
+  return dom::TreeColumns_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+XULTreeElement* nsTreeColumns::GetTree() const {
+  if (!mTree) {
+    return nullptr;
+  }
+
+  return XULTreeElement::FromNodeOrNull(mTree->GetBaseElement());
+}
+
+uint32_t nsTreeColumns::Count() {
+  EnsureColumns();
+  uint32_t count = 0;
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    ++count;
+  }
+  return count;
+}
+
+nsTreeColumn* nsTreeColumns::GetLastColumn() {
+  EnsureColumns();
+  nsTreeColumn* currCol = mFirstColumn;
+  while (currCol) {
+    nsTreeColumn* next = currCol->GetNext();
+    if (!next) {
+      return currCol;
+    }
+    currCol = next;
+  }
+  return nullptr;
+}
+
+nsTreeColumn* nsTreeColumns::GetSortedColumn() {
+  EnsureColumns();
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    if (nsContentUtils::HasNonEmptyAttr(currCol->mContent, kNameSpaceID_None,
+                                        nsGkAtoms::sortDirection)) {
+      return currCol;
+    }
+  }
+  return nullptr;
+}
+
+nsTreeColumn* nsTreeColumns::GetKeyColumn() {
+  EnsureColumns();
+
+  nsTreeColumn* first = nullptr;
+  nsTreeColumn* primary = nullptr;
+  nsTreeColumn* sorted = nullptr;
+
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    // Skip hidden columns.
+    if (currCol->mContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
+                                       nsGkAtoms::_true, eCaseMatters))
+      continue;
+
+    // Skip non-text column
+    if (currCol->GetType() != TreeColumn_Binding::TYPE_TEXT) continue;
+
+    if (!first) first = currCol;
+
+    if (nsContentUtils::HasNonEmptyAttr(currCol->mContent, kNameSpaceID_None,
+                                        nsGkAtoms::sortDirection)) {
+      // Use sorted column as the key.
+      sorted = currCol;
+      break;
+    }
+
+    if (currCol->IsPrimary())
+      if (!primary) primary = currCol;
+  }
+
+  if (sorted) return sorted;
+  if (primary) return primary;
+  return first;
+}
+
+nsTreeColumn* nsTreeColumns::GetColumnFor(dom::Element* aElement) {
+  EnsureColumns();
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    if (currCol->mContent == aElement) {
+      return currCol;
+    }
+  }
+  return nullptr;
+}
+
+nsTreeColumn* nsTreeColumns::NamedGetter(const nsAString& aId, bool& aFound) {
+  EnsureColumns();
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    if (currCol->GetId().Equals(aId)) {
+      aFound = true;
+      return currCol;
+    }
+  }
+  aFound = false;
+  return nullptr;
+}
+
+nsTreeColumn* nsTreeColumns::GetNamedColumn(const nsAString& aId) {
+  bool dummy;
+  return NamedGetter(aId, dummy);
+}
+
+void nsTreeColumns::GetSupportedNames(nsTArray<nsString>& aNames) {
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    aNames.AppendElement(currCol->GetId());
+  }
+}
+
+nsTreeColumn* nsTreeColumns::IndexedGetter(uint32_t aIndex, bool& aFound) {
+  EnsureColumns();
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    if (currCol->GetIndex() == static_cast<int32_t>(aIndex)) {
+      aFound = true;
+      return currCol;
+    }
+  }
+  aFound = false;
+  return nullptr;
+}
+
+nsTreeColumn* nsTreeColumns::GetColumnAt(uint32_t aIndex) {
+  bool dummy;
+  return IndexedGetter(aIndex, dummy);
+}
+
+void nsTreeColumns::InvalidateColumns() {
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    currCol->SetColumns(nullptr);
+  }
+  mFirstColumn = nullptr;
+}
+
+nsTreeColumn* nsTreeColumns::GetPrimaryColumn() {
+  EnsureColumns();
+  for (nsTreeColumn* currCol = mFirstColumn; currCol;
+       currCol = currCol->GetNext()) {
+    if (currCol->IsPrimary()) {
+      return currCol;
+    }
+  }
+  return nullptr;
+}
+
+void nsTreeColumns::EnsureColumns() {
+  if (mTree && !mFirstColumn) {
+    nsIContent* treeContent = mTree->GetBaseElement();
+    if (!treeContent) return;
+
+    nsIContent* colsContent =
+        nsTreeUtils::GetDescendantChild(treeContent, nsGkAtoms::treecols);
+    if (!colsContent) return;
+
+    nsIContent* colContent =
+        nsTreeUtils::GetDescendantChild(colsContent, nsGkAtoms::treecol);
+    if (!colContent) return;
+
+    nsIFrame* colFrame = colContent->GetPrimaryFrame();
+    if (!colFrame) return;
+
+    colFrame = colFrame->GetParent();
+    if (!colFrame || !colFrame->GetContent()) return;
+
+    nsTreeColumn* currCol = nullptr;
+
+    // Enumerate the columns in visible order
+    CSSOrderAwareFrameIterator iter(
+        colFrame, FrameChildListID::Principal,
+        CSSOrderAwareFrameIterator::ChildFilter::IncludeAll);
+    for (; !iter.AtEnd(); iter.Next()) {
+      nsIFrame* colFrame = iter.get();
+      nsIContent* colContent = colFrame->GetContent();
+      if (!colContent->IsXULElement(nsGkAtoms::treecol)) {
+        continue;
+      }
+      // Create a new column structure.
+      nsTreeColumn* col = new nsTreeColumn(this, colContent->AsElement());
+
+      if (currCol) {
+        currCol->SetNext(col);
+        col->SetPrevious(currCol);
+      } else {
+        mFirstColumn = col;
+      }
+      currCol = col;
+    }
+  }
+}
diff --git a/layout/xul/tree/nsTreeColumns.h b/layout/xul/tree/nsTreeColumns.h
new file mode 100644
index 0000000000..677206a89d
--- /dev/null
+++ b/layout/xul/tree/nsTreeColumns.h
@@ -0,0 +1,214 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeColumns_h__
+#define nsTreeColumns_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/RefPtr.h"
+#include "nsCoord.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsQueryObject.h"
+#include "nsWrapperCache.h"
+#include "nsString.h"
+
+class nsAtom;
+class nsTreeBodyFrame;
+class nsTreeColumns;
+class nsIFrame;
+class nsIContent;
+struct nsRect;
+
+namespace mozilla {
+enum class StyleTextAlignKeyword : uint8_t;
+using StyleTextAlign = StyleTextAlignKeyword;
+class ErrorResult;
+namespace dom {
+class Element;
+class XULTreeElement;
+}  // namespace dom
+}  // namespace mozilla
+
+#define NS_TREECOLUMN_IMPL_CID                       \
+  { /* 02cd1963-4b5d-4a6c-9223-814d3ade93a3 */       \
+    0x02cd1963, 0x4b5d, 0x4a6c, {                    \
+      0x92, 0x23, 0x81, 0x4d, 0x3a, 0xde, 0x93, 0xa3 \
+    }                                                \
+  }
+
+// This class is our column info.  We use it to iterate our columns and to
+// obtain information about each column.
+class nsTreeColumn final : public nsISupports, public nsWrapperCache {
+ public:
+  nsTreeColumn(nsTreeColumns* aColumns, mozilla::dom::Element* aElement);
+
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_TREECOLUMN_IMPL_CID)
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(nsTreeColumn)
+
+  // WebIDL
+  nsIContent* GetParentObject() const;
+  virtual JSObject* WrapObject(JSContext* aCx,
+                               JS::Handle<JSObject*> aGivenProto) override;
+
+  mozilla::dom::Element* Element();
+
+  nsTreeColumns* GetColumns() const { return mColumns; }
+
+  int32_t GetX(mozilla::ErrorResult& aRv);
+  int32_t GetWidth(mozilla::ErrorResult& aRv);
+
+  void GetId(nsAString& aId) const;
+  int32_t Index() const { return mIndex; }
+
+  bool Primary() const { return mIsPrimary; }
+  bool Cycler() const { return mIsCycler; }
+  bool Editable() const { return mIsEditable; }
+  int16_t Type() const { return mType; }
+
+  nsTreeColumn* GetNext() const { return mNext; }
+  nsTreeColumn* GetPrevious() const { return mPrevious; }
+
+  already_AddRefed<nsTreeColumn> GetPreviousColumn();
+
+  void Invalidate(mozilla::ErrorResult& aRv);
+
+  friend class nsTreeBodyFrame;
+  friend class nsTreeColumns;
+
+ protected:
+  ~nsTreeColumn();
+  nsIFrame* GetFrame();
+  nsIFrame* GetFrame(nsTreeBodyFrame* aBodyFrame);
+  // Don't call this if GetWidthInTwips or GetRect fails
+  bool IsLastVisible(nsTreeBodyFrame* aBodyFrame);
+
+  /**
+   * Returns a rect with x and width taken from the frame's rect and specified
+   * y and height. May fail in case there's no frame for the column.
+   */
+  nsresult GetRect(nsTreeBodyFrame* aBodyFrame, nscoord aY, nscoord aHeight,
+                   nsRect* aResult);
+
+  nsresult GetXInTwips(nsTreeBodyFrame* aBodyFrame, nscoord* aResult);
+  nsresult GetWidthInTwips(nsTreeBodyFrame* aBodyFrame, nscoord* aResult);
+
+  void SetColumns(nsTreeColumns* aColumns) { mColumns = aColumns; }
+
+ public:
+  const nsAString& GetId() const { return mId; }
+  nsAtom* GetAtom() { return mAtom; }
+  int32_t GetIndex() { return mIndex; }
+
+ protected:
+  bool IsPrimary() { return mIsPrimary; }
+  bool IsCycler() { return mIsCycler; }
+  bool IsEditable() { return mIsEditable; }
+  bool Overflow() { return mOverflow; }
+
+  int16_t GetType() { return mType; }
+
+  int8_t GetCropStyle() { return mCropStyle; }
+  mozilla::StyleTextAlign GetTextAlignment() { return mTextAlignment; }
+
+  void SetNext(nsTreeColumn* aNext) {
+    NS_ASSERTION(!mNext, "already have a next sibling");
+    mNext = aNext;
+  }
+  void SetPrevious(nsTreeColumn* aPrevious) { mPrevious = aPrevious; }
+
+ private:
+  /**
+   * Non-null nsIContent for the associated <treecol> element.
+   */
+  RefPtr<mozilla::dom::Element> mContent;
+
+  nsTreeColumns* mColumns;
+
+  nsString mId;
+  RefPtr<nsAtom> mAtom;
+
+  int32_t mIndex;
+
+  bool mIsPrimary;
+  bool mIsCycler;
+  bool mIsEditable;
+  bool mOverflow;
+
+  int16_t mType;
+
+  int8_t mCropStyle;
+  mozilla::StyleTextAlign mTextAlignment;
+
+  RefPtr<nsTreeColumn> mNext;
+  nsTreeColumn* mPrevious;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsTreeColumn, NS_TREECOLUMN_IMPL_CID)
+
+class nsTreeColumns final : public nsISupports, public nsWrapperCache {
+ private:
+  ~nsTreeColumns();
+
+ public:
+  explicit nsTreeColumns(nsTreeBodyFrame* aTree);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(nsTreeColumns)
+
+  nsIContent* GetParentObject() const;
+  virtual JSObject* WrapObject(JSContext* aCx,
+                               JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL
+  mozilla::dom::XULTreeElement* GetTree() const;
+  uint32_t Count();
+  uint32_t Length() { return Count(); }
+
+  nsTreeColumn* GetFirstColumn() {
+    EnsureColumns();
+    return mFirstColumn;
+  }
+  nsTreeColumn* GetLastColumn();
+
+  nsTreeColumn* GetPrimaryColumn();
+  nsTreeColumn* GetSortedColumn();
+  nsTreeColumn* GetKeyColumn();
+
+  nsTreeColumn* GetColumnFor(mozilla::dom::Element* aElement);
+
+  nsTreeColumn* IndexedGetter(uint32_t aIndex, bool& aFound);
+  nsTreeColumn* GetColumnAt(uint32_t aIndex);
+  nsTreeColumn* NamedGetter(const nsAString& aId, bool& aFound);
+  nsTreeColumn* GetNamedColumn(const nsAString& aId);
+  void GetSupportedNames(nsTArray<nsString>& aNames);
+
+  void InvalidateColumns();
+
+  friend class nsTreeBodyFrame;
+
+ protected:
+  void SetTree(nsTreeBodyFrame* aTree) { mTree = aTree; }
+
+  // Builds our cache of column info.
+  void EnsureColumns();
+
+ private:
+  nsTreeBodyFrame* mTree;
+
+  /**
+   * The first column in the list of columns. All of the columns are supposed
+   * to be "alive", i.e. have a frame. This is achieved by clearing the columns
+   * list each time a treecol changes size.
+   *
+   * XXX this means that new nsTreeColumn objects are unnecessarily created
+   *     for untouched columns.
+   */
+  RefPtr<nsTreeColumn> mFirstColumn;
+};
+
+#endif  // nsTreeColumns_h__
diff --git a/layout/xul/tree/nsTreeContentView.cpp b/layout/xul/tree/nsTreeContentView.cpp
new file mode 100644
index 0000000000..edb5368e2a
--- /dev/null
+++ b/layout/xul/tree/nsTreeContentView.cpp
@@ -0,0 +1,1269 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsNameSpaceManager.h"
+#include "nsGkAtoms.h"
+#include "nsTreeUtils.h"
+#include "nsTreeContentView.h"
+#include "ChildIterator.h"
+#include "nsError.h"
+#include "nsXULSortService.h"
+#include "nsTreeBodyFrame.h"
+#include "nsTreeColumns.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/TreeContentViewBinding.h"
+#include "mozilla/dom/XULTreeElement.h"
+#include "nsServiceManagerUtils.h"
+#include "mozilla/dom/Document.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+// A content model view implementation for the tree.
+
+#define ROW_FLAG_CONTAINER 0x01
+#define ROW_FLAG_OPEN 0x02
+#define ROW_FLAG_EMPTY 0x04
+#define ROW_FLAG_SEPARATOR 0x08
+
+class Row {
+ public:
+  Row(Element* aContent, int32_t aParentIndex)
+      : mContent(aContent),
+        mParentIndex(aParentIndex),
+        mSubtreeSize(0),
+        mFlags(0) {}
+
+  ~Row() = default;
+
+  void SetContainer(bool aContainer) {
+    aContainer ? mFlags |= ROW_FLAG_CONTAINER : mFlags &= ~ROW_FLAG_CONTAINER;
+  }
+  bool IsContainer() { return mFlags & ROW_FLAG_CONTAINER; }
+
+  void SetOpen(bool aOpen) {
+    aOpen ? mFlags |= ROW_FLAG_OPEN : mFlags &= ~ROW_FLAG_OPEN;
+  }
+  bool IsOpen() { return !!(mFlags & ROW_FLAG_OPEN); }
+
+  void SetEmpty(bool aEmpty) {
+    aEmpty ? mFlags |= ROW_FLAG_EMPTY : mFlags &= ~ROW_FLAG_EMPTY;
+  }
+  bool IsEmpty() { return !!(mFlags & ROW_FLAG_EMPTY); }
+
+  void SetSeparator(bool aSeparator) {
+    aSeparator ? mFlags |= ROW_FLAG_SEPARATOR : mFlags &= ~ROW_FLAG_SEPARATOR;
+  }
+  bool IsSeparator() { return !!(mFlags & ROW_FLAG_SEPARATOR); }
+
+  // Weak reference to a content item.
+  Element* mContent;
+
+  // The parent index of the item, set to -1 for the top level items.
+  int32_t mParentIndex;
+
+  // Subtree size for this item.
+  int32_t mSubtreeSize;
+
+ private:
+  // State flags
+  int8_t mFlags;
+};
+
+// We don't reference count the reference to the document
+// If the document goes away first, we'll be informed and we
+// can drop our reference.
+// If we go away first, we'll get rid of ourselves from the
+// document's observer list.
+
+nsTreeContentView::nsTreeContentView(void)
+    : mTree(nullptr), mSelection(nullptr), mDocument(nullptr) {}
+
+nsTreeContentView::~nsTreeContentView(void) {
+  // Remove ourselves from mDocument's observers.
+  if (mDocument) mDocument->RemoveObserver(this);
+}
+
+nsresult NS_NewTreeContentView(nsITreeView** aResult) {
+  *aResult = new nsTreeContentView;
+  if (!*aResult) return NS_ERROR_OUT_OF_MEMORY;
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(nsTreeContentView, mTree, mSelection,
+                                      mBody)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsTreeContentView)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsTreeContentView)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsTreeContentView)
+  NS_INTERFACE_MAP_ENTRY(nsITreeView)
+  NS_INTERFACE_MAP_ENTRY(nsIDocumentObserver)
+  NS_INTERFACE_MAP_ENTRY(nsIMutationObserver)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsITreeView)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END
+
+JSObject* nsTreeContentView::WrapObject(JSContext* aCx,
+                                        JS::Handle<JSObject*> aGivenProto) {
+  return TreeContentView_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+nsISupports* nsTreeContentView::GetParentObject() { return mTree; }
+
+NS_IMETHODIMP
+nsTreeContentView::GetRowCount(int32_t* aRowCount) {
+  *aRowCount = mRows.Length();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetSelection(nsITreeSelection** aSelection) {
+  NS_IF_ADDREF(*aSelection = GetSelection());
+
+  return NS_OK;
+}
+
+bool nsTreeContentView::CanTrustTreeSelection(nsISupports* aValue) {
+  // Untrusted content is only allowed to specify known-good views
+  if (nsContentUtils::LegacyIsCallerChromeOrNativeCode()) return true;
+  nsCOMPtr<nsINativeTreeSelection> nativeTreeSel = do_QueryInterface(aValue);
+  return nativeTreeSel && NS_SUCCEEDED(nativeTreeSel->EnsureNative());
+}
+
+NS_IMETHODIMP
+nsTreeContentView::SetSelection(nsITreeSelection* aSelection) {
+  ErrorResult rv;
+  SetSelection(aSelection, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::SetSelection(nsITreeSelection* aSelection,
+                                     ErrorResult& aError) {
+  if (aSelection && !CanTrustTreeSelection(aSelection)) {
+    aError.ThrowSecurityError("Not allowed to set tree selection");
+    return;
+  }
+
+  mSelection = aSelection;
+}
+
+void nsTreeContentView::GetRowProperties(int32_t aRow, nsAString& aProperties,
+                                         ErrorResult& aError) {
+  aProperties.Truncate();
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+  nsIContent* realRow;
+  if (row->IsSeparator())
+    realRow = row->mContent;
+  else
+    realRow = nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+
+  if (realRow && realRow->IsElement()) {
+    realRow->AsElement()->GetAttr(kNameSpaceID_None, nsGkAtoms::properties,
+                                  aProperties);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetRowProperties(int32_t aIndex, nsAString& aProps) {
+  ErrorResult rv;
+  GetRowProperties(aIndex, aProps, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::GetCellProperties(int32_t aRow, nsTreeColumn& aColumn,
+                                          nsAString& aProperties,
+                                          ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+  nsIContent* realRow =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+  if (realRow) {
+    Element* cell = GetCell(realRow, aColumn);
+    if (cell) {
+      cell->GetAttr(kNameSpaceID_None, nsGkAtoms::properties, aProperties);
+    }
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetCellProperties(int32_t aRow, nsTreeColumn* aCol,
+                                     nsAString& aProps) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  GetCellProperties(aRow, *aCol, aProps, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::GetColumnProperties(nsTreeColumn& aColumn,
+                                            nsAString& aProperties) {
+  RefPtr<Element> element = aColumn.Element();
+
+  if (element) {
+    element->GetAttr(nsGkAtoms::properties, aProperties);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetColumnProperties(nsTreeColumn* aCol, nsAString& aProps) {
+  NS_ENSURE_ARG(aCol);
+
+  GetColumnProperties(*aCol, aProps);
+  return NS_OK;
+}
+
+bool nsTreeContentView::IsContainer(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return false;
+  }
+
+  return mRows[aRow]->IsContainer();
+}
+
+NS_IMETHODIMP
+nsTreeContentView::IsContainer(int32_t aIndex, bool* _retval) {
+  ErrorResult rv;
+  *_retval = IsContainer(aIndex, rv);
+  return rv.StealNSResult();
+}
+
+bool nsTreeContentView::IsContainerOpen(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return false;
+  }
+
+  return mRows[aRow]->IsOpen();
+}
+
+NS_IMETHODIMP
+nsTreeContentView::IsContainerOpen(int32_t aIndex, bool* _retval) {
+  ErrorResult rv;
+  *_retval = IsContainerOpen(aIndex, rv);
+  return rv.StealNSResult();
+}
+
+bool nsTreeContentView::IsContainerEmpty(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return false;
+  }
+
+  return mRows[aRow]->IsEmpty();
+}
+
+NS_IMETHODIMP
+nsTreeContentView::IsContainerEmpty(int32_t aIndex, bool* _retval) {
+  ErrorResult rv;
+  *_retval = IsContainerEmpty(aIndex, rv);
+  return rv.StealNSResult();
+}
+
+bool nsTreeContentView::IsSeparator(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return false;
+  }
+
+  return mRows[aRow]->IsSeparator();
+}
+
+NS_IMETHODIMP
+nsTreeContentView::IsSeparator(int32_t aIndex, bool* _retval) {
+  ErrorResult rv;
+  *_retval = IsSeparator(aIndex, rv);
+  return rv.StealNSResult();
+}
+
+NS_IMETHODIMP
+nsTreeContentView::IsSorted(bool* _retval) {
+  *_retval = IsSorted();
+
+  return NS_OK;
+}
+
+bool nsTreeContentView::CanDrop(int32_t aRow, int32_t aOrientation,
+                                ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+  }
+  return false;
+}
+
+bool nsTreeContentView::CanDrop(int32_t aRow, int32_t aOrientation,
+                                DataTransfer* aDataTransfer,
+                                ErrorResult& aError) {
+  return CanDrop(aRow, aOrientation, aError);
+}
+
+NS_IMETHODIMP
+nsTreeContentView::CanDrop(int32_t aIndex, int32_t aOrientation,
+                           DataTransfer* aDataTransfer, bool* _retval) {
+  ErrorResult rv;
+  *_retval = CanDrop(aIndex, aOrientation, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::Drop(int32_t aRow, int32_t aOrientation,
+                             ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+  }
+}
+
+void nsTreeContentView::Drop(int32_t aRow, int32_t aOrientation,
+                             DataTransfer* aDataTransfer, ErrorResult& aError) {
+  Drop(aRow, aOrientation, aError);
+}
+
+NS_IMETHODIMP
+nsTreeContentView::Drop(int32_t aRow, int32_t aOrientation,
+                        DataTransfer* aDataTransfer) {
+  ErrorResult rv;
+  Drop(aRow, aOrientation, rv);
+  return rv.StealNSResult();
+}
+
+int32_t nsTreeContentView::GetParentIndex(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return 0;
+  }
+
+  return mRows[aRow]->mParentIndex;
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetParentIndex(int32_t aRowIndex, int32_t* _retval) {
+  ErrorResult rv;
+  *_retval = GetParentIndex(aRowIndex, rv);
+  return rv.StealNSResult();
+}
+
+bool nsTreeContentView::HasNextSibling(int32_t aRow, int32_t aAfterIndex,
+                                       ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return false;
+  }
+
+  // We have a next sibling if the row is not the last in the subtree.
+  int32_t parentIndex = mRows[aRow]->mParentIndex;
+  if (parentIndex < 0) {
+    return uint32_t(aRow) < mRows.Length() - 1;
+  }
+
+  // Compute the last index in this subtree.
+  int32_t lastIndex = parentIndex + (mRows[parentIndex])->mSubtreeSize;
+  Row* row = mRows[lastIndex].get();
+  while (row->mParentIndex != parentIndex) {
+    lastIndex = row->mParentIndex;
+    row = mRows[lastIndex].get();
+  }
+
+  return aRow < lastIndex;
+}
+
+NS_IMETHODIMP
+nsTreeContentView::HasNextSibling(int32_t aRowIndex, int32_t aAfterIndex,
+                                  bool* _retval) {
+  ErrorResult rv;
+  *_retval = HasNextSibling(aRowIndex, aAfterIndex, rv);
+  return rv.StealNSResult();
+}
+
+int32_t nsTreeContentView::GetLevel(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return 0;
+  }
+
+  int32_t level = 0;
+  Row* row = mRows[aRow].get();
+  while (row->mParentIndex >= 0) {
+    level++;
+    row = mRows[row->mParentIndex].get();
+  }
+  return level;
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetLevel(int32_t aIndex, int32_t* _retval) {
+  ErrorResult rv;
+  *_retval = GetLevel(aIndex, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::GetImageSrc(int32_t aRow, nsTreeColumn& aColumn,
+                                    nsAString& aSrc, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+
+  nsIContent* realRow =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+  if (realRow) {
+    Element* cell = GetCell(realRow, aColumn);
+    if (cell) cell->GetAttr(kNameSpaceID_None, nsGkAtoms::src, aSrc);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetImageSrc(int32_t aRow, nsTreeColumn* aCol,
+                               nsAString& _retval) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  GetImageSrc(aRow, *aCol, _retval, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::GetCellValue(int32_t aRow, nsTreeColumn& aColumn,
+                                     nsAString& aValue, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+
+  nsIContent* realRow =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+  if (realRow) {
+    Element* cell = GetCell(realRow, aColumn);
+    if (cell) cell->GetAttr(kNameSpaceID_None, nsGkAtoms::value, aValue);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetCellValue(int32_t aRow, nsTreeColumn* aCol,
+                                nsAString& _retval) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  GetCellValue(aRow, *aCol, _retval, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::GetCellText(int32_t aRow, nsTreeColumn& aColumn,
+                                    nsAString& aText, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+
+  // Check for a "label" attribute - this is valid on an <treeitem>
+  // with a single implied column.
+  if (row->mContent->GetAttr(kNameSpaceID_None, nsGkAtoms::label, aText) &&
+      !aText.IsEmpty()) {
+    return;
+  }
+
+  if (row->mContent->IsXULElement(nsGkAtoms::treeitem)) {
+    nsIContent* realRow =
+        nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+    if (realRow) {
+      Element* cell = GetCell(realRow, aColumn);
+      if (cell) cell->GetAttr(kNameSpaceID_None, nsGkAtoms::label, aText);
+    }
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::GetCellText(int32_t aRow, nsTreeColumn* aCol,
+                               nsAString& _retval) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  GetCellText(aRow, *aCol, _retval, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::SetTree(XULTreeElement* aTree, ErrorResult& aError) {
+  aError = SetTree(aTree);
+}
+
+NS_IMETHODIMP
+nsTreeContentView::SetTree(XULTreeElement* aTree) {
+  ClearRows();
+
+  mTree = aTree;
+
+  if (aTree) {
+    // Add ourselves to document's observers.
+    Document* document = mTree->GetComposedDoc();
+    if (document) {
+      document->AddObserver(this);
+      mDocument = document;
+    }
+
+    RefPtr<dom::Element> bodyElement = mTree->GetTreeBody();
+    if (bodyElement) {
+      mBody = std::move(bodyElement);
+      int32_t index = 0;
+      Serialize(mBody, -1, &index, mRows);
+    }
+  }
+
+  return NS_OK;
+}
+
+void nsTreeContentView::ToggleOpenState(int32_t aRow, ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  // We don't serialize content right here, since content might be generated
+  // lazily.
+  Row* row = mRows[aRow].get();
+
+  if (row->IsOpen())
+    row->mContent->SetAttr(kNameSpaceID_None, nsGkAtoms::open, u"false"_ns,
+                           true);
+  else
+    row->mContent->SetAttr(kNameSpaceID_None, nsGkAtoms::open, u"true"_ns,
+                           true);
+}
+
+NS_IMETHODIMP
+nsTreeContentView::ToggleOpenState(int32_t aIndex) {
+  ErrorResult rv;
+  ToggleOpenState(aIndex, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::CycleHeader(nsTreeColumn& aColumn,
+                                    ErrorResult& aError) {
+  if (!mTree) return;
+
+  RefPtr<Element> column = aColumn.Element();
+  nsAutoString sort;
+  column->GetAttr(kNameSpaceID_None, nsGkAtoms::sort, sort);
+  if (!sort.IsEmpty()) {
+    nsAutoString sortdirection;
+    static Element::AttrValuesArray strings[] = {
+        nsGkAtoms::ascending, nsGkAtoms::descending, nullptr};
+    switch (column->FindAttrValueIn(kNameSpaceID_None, nsGkAtoms::sortDirection,
+                                    strings, eCaseMatters)) {
+      case 0:
+        sortdirection.AssignLiteral("descending");
+        break;
+      case 1:
+        sortdirection.AssignLiteral("natural");
+        break;
+      default:
+        sortdirection.AssignLiteral("ascending");
+        break;
+    }
+
+    nsAutoString hints;
+    column->GetAttr(kNameSpaceID_None, nsGkAtoms::sorthints, hints);
+    sortdirection.Append(' ');
+    sortdirection += hints;
+
+    XULWidgetSort(mTree, sort, sortdirection);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::CycleHeader(nsTreeColumn* aCol) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  CycleHeader(*aCol, rv);
+  return rv.StealNSResult();
+}
+
+NS_IMETHODIMP
+nsTreeContentView::SelectionChangedXPCOM() { return NS_OK; }
+
+NS_IMETHODIMP
+nsTreeContentView::CycleCell(int32_t aRow, nsTreeColumn* aCol) { return NS_OK; }
+
+bool nsTreeContentView::IsEditable(int32_t aRow, nsTreeColumn& aColumn,
+                                   ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return false;
+  }
+
+  Row* row = mRows[aRow].get();
+
+  nsIContent* realRow =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+  if (realRow) {
+    Element* cell = GetCell(realRow, aColumn);
+    if (cell && cell->AttrValueIs(kNameSpaceID_None, nsGkAtoms::editable,
+                                  nsGkAtoms::_false, eCaseMatters)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+NS_IMETHODIMP
+nsTreeContentView::IsEditable(int32_t aRow, nsTreeColumn* aCol, bool* _retval) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  *_retval = IsEditable(aRow, *aCol, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::SetCellValue(int32_t aRow, nsTreeColumn& aColumn,
+                                     const nsAString& aValue,
+                                     ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+
+  nsIContent* realRow =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+  if (realRow) {
+    Element* cell = GetCell(realRow, aColumn);
+    if (cell) cell->SetAttr(kNameSpaceID_None, nsGkAtoms::value, aValue, true);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::SetCellValue(int32_t aRow, nsTreeColumn* aCol,
+                                const nsAString& aValue) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  SetCellValue(aRow, *aCol, aValue, rv);
+  return rv.StealNSResult();
+}
+
+void nsTreeContentView::SetCellText(int32_t aRow, nsTreeColumn& aColumn,
+                                    const nsAString& aValue,
+                                    ErrorResult& aError) {
+  if (!IsValidRowIndex(aRow)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return;
+  }
+
+  Row* row = mRows[aRow].get();
+
+  nsIContent* realRow =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treerow);
+  if (realRow) {
+    Element* cell = GetCell(realRow, aColumn);
+    if (cell) cell->SetAttr(kNameSpaceID_None, nsGkAtoms::label, aValue, true);
+  }
+}
+
+NS_IMETHODIMP
+nsTreeContentView::SetCellText(int32_t aRow, nsTreeColumn* aCol,
+                               const nsAString& aValue) {
+  NS_ENSURE_ARG(aCol);
+
+  ErrorResult rv;
+  SetCellText(aRow, *aCol, aValue, rv);
+  return rv.StealNSResult();
+}
+
+Element* nsTreeContentView::GetItemAtIndex(int32_t aIndex,
+                                           ErrorResult& aError) {
+  if (!IsValidRowIndex(aIndex)) {
+    aError.Throw(NS_ERROR_INVALID_ARG);
+    return nullptr;
+  }
+
+  return mRows[aIndex]->mContent;
+}
+
+int32_t nsTreeContentView::GetIndexOfItem(Element* aItem) {
+  return FindContent(aItem);
+}
+
+void nsTreeContentView::AttributeChanged(dom::Element* aElement,
+                                         int32_t aNameSpaceID,
+                                         nsAtom* aAttribute, int32_t aModType,
+                                         const nsAttrValue* aOldValue) {
+  // Lots of codepaths under here that do all sorts of stuff, so be safe.
+  nsCOMPtr<nsIMutationObserver> kungFuDeathGrip(this);
+
+  // Make sure this notification concerns us.
+  // First check the tag to see if it's one that we care about.
+  if (aElement == mTree || aElement == mBody) {
+    mTree->ClearStyleAndImageCaches();
+    mTree->Invalidate();
+  }
+
+  // We don't consider non-XUL nodes.
+  nsIContent* parent = nullptr;
+  if (!aElement->IsXULElement() ||
+      ((parent = aElement->GetParent()) && !parent->IsXULElement())) {
+    return;
+  }
+  if (!aElement->IsAnyOfXULElements(nsGkAtoms::treecol, nsGkAtoms::treeitem,
+                                    nsGkAtoms::treeseparator,
+                                    nsGkAtoms::treerow, nsGkAtoms::treecell)) {
+    return;
+  }
+
+  // If we have a legal tag, go up to the tree/select and make sure
+  // that it's ours.
+
+  for (nsIContent* element = aElement; element != mBody;
+       element = element->GetParent()) {
+    if (!element) return;                                // this is not for us
+    if (element->IsXULElement(nsGkAtoms::tree)) return;  // this is not for us
+  }
+
+  // Handle changes of the hidden attribute.
+  if (aAttribute == nsGkAtoms::hidden &&
+      aElement->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                   nsGkAtoms::treeseparator)) {
+    bool hidden = aElement->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
+                                        nsGkAtoms::_true, eCaseMatters);
+
+    int32_t index = FindContent(aElement);
+    if (hidden && index >= 0) {
+      // Hide this row along with its children.
+      int32_t count = RemoveRow(index);
+      if (mTree) mTree->RowCountChanged(index, -count);
+    } else if (!hidden && index < 0) {
+      // Show this row along with its children.
+      nsCOMPtr<nsIContent> parent = aElement->GetParent();
+      if (parent) {
+        InsertRowFor(parent, aElement);
+      }
+    }
+
+    return;
+  }
+
+  if (aElement->IsXULElement(nsGkAtoms::treecol)) {
+    if (aAttribute == nsGkAtoms::properties) {
+      if (mTree) {
+        RefPtr<nsTreeColumns> cols = mTree->GetColumns();
+        if (cols) {
+          RefPtr<nsTreeColumn> col = cols->GetColumnFor(aElement);
+          mTree->InvalidateColumn(col);
+        }
+      }
+    }
+  } else if (aElement->IsXULElement(nsGkAtoms::treeitem)) {
+    int32_t index = FindContent(aElement);
+    if (index >= 0) {
+      Row* row = mRows[index].get();
+      if (aAttribute == nsGkAtoms::container) {
+        bool isContainer =
+            aElement->AttrValueIs(kNameSpaceID_None, nsGkAtoms::container,
+                                  nsGkAtoms::_true, eCaseMatters);
+        row->SetContainer(isContainer);
+        if (mTree) mTree->InvalidateRow(index);
+      } else if (aAttribute == nsGkAtoms::open) {
+        bool isOpen = aElement->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
+                                            nsGkAtoms::_true, eCaseMatters);
+        bool wasOpen = row->IsOpen();
+        if (!isOpen && wasOpen)
+          CloseContainer(index);
+        else if (isOpen && !wasOpen)
+          OpenContainer(index);
+      } else if (aAttribute == nsGkAtoms::empty) {
+        bool isEmpty =
+            aElement->AttrValueIs(kNameSpaceID_None, nsGkAtoms::empty,
+                                  nsGkAtoms::_true, eCaseMatters);
+        row->SetEmpty(isEmpty);
+        if (mTree) mTree->InvalidateRow(index);
+      }
+    }
+  } else if (aElement->IsXULElement(nsGkAtoms::treeseparator)) {
+    int32_t index = FindContent(aElement);
+    if (index >= 0) {
+      if (aAttribute == nsGkAtoms::properties && mTree) {
+        mTree->InvalidateRow(index);
+      }
+    }
+  } else if (aElement->IsXULElement(nsGkAtoms::treerow)) {
+    if (aAttribute == nsGkAtoms::properties) {
+      nsCOMPtr<nsIContent> parent = aElement->GetParent();
+      if (parent) {
+        int32_t index = FindContent(parent);
+        if (index >= 0 && mTree) {
+          mTree->InvalidateRow(index);
+        }
+      }
+    }
+  } else if (aElement->IsXULElement(nsGkAtoms::treecell)) {
+    if (aAttribute == nsGkAtoms::properties || aAttribute == nsGkAtoms::mode ||
+        aAttribute == nsGkAtoms::src || aAttribute == nsGkAtoms::value ||
+        aAttribute == nsGkAtoms::label) {
+      nsIContent* parent = aElement->GetParent();
+      if (parent) {
+        nsCOMPtr<nsIContent> grandParent = parent->GetParent();
+        if (grandParent && grandParent->IsXULElement()) {
+          int32_t index = FindContent(grandParent);
+          if (index >= 0 && mTree) {
+            // XXX Should we make an effort to invalidate only cell ?
+            mTree->InvalidateRow(index);
+          }
+        }
+      }
+    }
+  }
+}
+
+void nsTreeContentView::ContentAppended(nsIContent* aFirstNewContent) {
+  for (nsIContent* cur = aFirstNewContent; cur; cur = cur->GetNextSibling()) {
+    // Our contentinserted doesn't use the index
+    ContentInserted(cur);
+  }
+}
+
+void nsTreeContentView::ContentInserted(nsIContent* aChild) {
+  NS_ASSERTION(aChild, "null ptr");
+  nsIContent* container = aChild->GetParent();
+
+  // Make sure this notification concerns us.
+  // First check the tag to see if it's one that we care about.
+
+  // Don't allow non-XUL nodes.
+  if (!aChild->IsXULElement() || !container->IsXULElement()) return;
+
+  if (!aChild->IsAnyOfXULElements(nsGkAtoms::treeitem, nsGkAtoms::treeseparator,
+                                  nsGkAtoms::treechildren, nsGkAtoms::treerow,
+                                  nsGkAtoms::treecell)) {
+    return;
+  }
+
+  // If we have a legal tag, go up to the tree/select and make sure
+  // that it's ours.
+
+  for (nsIContent* element = container; element != mBody;
+       element = element->GetParent()) {
+    if (!element) return;                                // this is not for us
+    if (element->IsXULElement(nsGkAtoms::tree)) return;  // this is not for us
+  }
+
+  // Lots of codepaths under here that do all sorts of stuff, so be safe.
+  nsCOMPtr<nsIMutationObserver> kungFuDeathGrip(this);
+
+  if (aChild->IsXULElement(nsGkAtoms::treechildren)) {
+    int32_t index = FindContent(container);
+    if (index >= 0) {
+      Row* row = mRows[index].get();
+      row->SetEmpty(false);
+      if (mTree) mTree->InvalidateRow(index);
+      if (row->IsContainer() && row->IsOpen()) {
+        int32_t count = EnsureSubtree(index);
+        if (mTree) mTree->RowCountChanged(index + 1, count);
+      }
+    }
+  } else if (aChild->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                        nsGkAtoms::treeseparator)) {
+    InsertRowFor(container, aChild);
+  } else if (aChild->IsXULElement(nsGkAtoms::treerow)) {
+    int32_t index = FindContent(container);
+    if (index >= 0 && mTree) mTree->InvalidateRow(index);
+  } else if (aChild->IsXULElement(nsGkAtoms::treecell)) {
+    nsCOMPtr<nsIContent> parent = container->GetParent();
+    if (parent) {
+      int32_t index = FindContent(parent);
+      if (index >= 0 && mTree) mTree->InvalidateRow(index);
+    }
+  }
+}
+
+void nsTreeContentView::ContentRemoved(nsIContent* aChild,
+                                       nsIContent* aPreviousSibling) {
+  NS_ASSERTION(aChild, "null ptr");
+
+  nsIContent* container = aChild->GetParent();
+  // Make sure this notification concerns us.
+  // First check the tag to see if it's one that we care about.
+
+  // We don't consider non-XUL nodes.
+  if (!aChild->IsXULElement() || !container->IsXULElement()) return;
+
+  if (!aChild->IsAnyOfXULElements(nsGkAtoms::treeitem, nsGkAtoms::treeseparator,
+                                  nsGkAtoms::treechildren, nsGkAtoms::treerow,
+                                  nsGkAtoms::treecell)) {
+    return;
+  }
+
+  // If we have a legal tag, go up to the tree/select and make sure
+  // that it's ours.
+
+  for (nsIContent* element = container; element != mBody;
+       element = element->GetParent()) {
+    if (!element) return;                                // this is not for us
+    if (element->IsXULElement(nsGkAtoms::tree)) return;  // this is not for us
+  }
+
+  // Lots of codepaths under here that do all sorts of stuff, so be safe.
+  nsCOMPtr<nsIMutationObserver> kungFuDeathGrip(this);
+
+  if (aChild->IsXULElement(nsGkAtoms::treechildren)) {
+    int32_t index = FindContent(container);
+    if (index >= 0) {
+      Row* row = mRows[index].get();
+      row->SetEmpty(true);
+      int32_t count = RemoveSubtree(index);
+      // Invalidate also the row to update twisty.
+      if (mTree) {
+        mTree->InvalidateRow(index);
+        mTree->RowCountChanged(index + 1, -count);
+      }
+    }
+  } else if (aChild->IsAnyOfXULElements(nsGkAtoms::treeitem,
+                                        nsGkAtoms::treeseparator)) {
+    int32_t index = FindContent(aChild);
+    if (index >= 0) {
+      int32_t count = RemoveRow(index);
+      if (mTree) mTree->RowCountChanged(index, -count);
+    }
+  } else if (aChild->IsXULElement(nsGkAtoms::treerow)) {
+    int32_t index = FindContent(container);
+    if (index >= 0 && mTree) mTree->InvalidateRow(index);
+  } else if (aChild->IsXULElement(nsGkAtoms::treecell)) {
+    nsCOMPtr<nsIContent> parent = container->GetParent();
+    if (parent) {
+      int32_t index = FindContent(parent);
+      if (index >= 0 && mTree) mTree->InvalidateRow(index);
+    }
+  }
+}
+
+void nsTreeContentView::NodeWillBeDestroyed(nsINode* aNode) {
+  // XXXbz do we need this strong ref?  Do we drop refs to self in ClearRows?
+  nsCOMPtr<nsIMutationObserver> kungFuDeathGrip(this);
+  ClearRows();
+}
+
+// Recursively serialize content, starting with aContent.
+void nsTreeContentView::Serialize(nsIContent* aContent, int32_t aParentIndex,
+                                  int32_t* aIndex,
+                                  nsTArray<UniquePtr<Row>>& aRows) {
+  // Don't allow non-XUL nodes.
+  if (!aContent->IsXULElement()) return;
+
+  dom::FlattenedChildIterator iter(aContent);
+  for (nsIContent* content = iter.GetNextChild(); content;
+       content = iter.GetNextChild()) {
+    int32_t count = aRows.Length();
+
+    if (content->IsXULElement(nsGkAtoms::treeitem)) {
+      SerializeItem(content->AsElement(), aParentIndex, aIndex, aRows);
+    } else if (content->IsXULElement(nsGkAtoms::treeseparator)) {
+      SerializeSeparator(content->AsElement(), aParentIndex, aIndex, aRows);
+    }
+
+    *aIndex += aRows.Length() - count;
+  }
+}
+
+void nsTreeContentView::SerializeItem(Element* aContent, int32_t aParentIndex,
+                                      int32_t* aIndex,
+                                      nsTArray<UniquePtr<Row>>& aRows) {
+  if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
+                            nsGkAtoms::_true, eCaseMatters))
+    return;
+
+  aRows.AppendElement(MakeUnique<Row>(aContent, aParentIndex));
+  Row* row = aRows.LastElement().get();
+
+  if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::container,
+                            nsGkAtoms::_true, eCaseMatters)) {
+    row->SetContainer(true);
+    if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::open,
+                              nsGkAtoms::_true, eCaseMatters)) {
+      row->SetOpen(true);
+      nsIContent* child =
+          nsTreeUtils::GetImmediateChild(aContent, nsGkAtoms::treechildren);
+      if (child && child->IsXULElement()) {
+        // Now, recursively serialize our child.
+        int32_t count = aRows.Length();
+        int32_t index = 0;
+        Serialize(child, aParentIndex + *aIndex + 1, &index, aRows);
+        row->mSubtreeSize += aRows.Length() - count;
+      } else
+        row->SetEmpty(true);
+    } else if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::empty,
+                                     nsGkAtoms::_true, eCaseMatters)) {
+      row->SetEmpty(true);
+    }
+  }
+}
+
+void nsTreeContentView::SerializeSeparator(Element* aContent,
+                                           int32_t aParentIndex,
+                                           int32_t* aIndex,
+                                           nsTArray<UniquePtr<Row>>& aRows) {
+  if (aContent->AttrValueIs(kNameSpaceID_None, nsGkAtoms::hidden,
+                            nsGkAtoms::_true, eCaseMatters))
+    return;
+
+  auto row = MakeUnique<Row>(aContent, aParentIndex);
+  row->SetSeparator(true);
+  aRows.AppendElement(std::move(row));
+}
+
+void nsTreeContentView::GetIndexInSubtree(nsIContent* aContainer,
+                                          nsIContent* aContent,
+                                          int32_t* aIndex) {
+  if (!aContainer->IsXULElement()) return;
+
+  for (nsIContent* content = aContainer->GetFirstChild(); content;
+       content = content->GetNextSibling()) {
+    if (content == aContent) break;
+
+    if (content->IsXULElement(nsGkAtoms::treeitem)) {
+      if (!content->AsElement()->AttrValueIs(kNameSpaceID_None,
+                                             nsGkAtoms::hidden,
+                                             nsGkAtoms::_true, eCaseMatters)) {
+        (*aIndex)++;
+        if (content->AsElement()->AttrValueIs(kNameSpaceID_None,
+                                              nsGkAtoms::container,
+                                              nsGkAtoms::_true, eCaseMatters) &&
+            content->AsElement()->AttrValueIs(kNameSpaceID_None,
+                                              nsGkAtoms::open, nsGkAtoms::_true,
+                                              eCaseMatters)) {
+          nsIContent* child =
+              nsTreeUtils::GetImmediateChild(content, nsGkAtoms::treechildren);
+          if (child && child->IsXULElement())
+            GetIndexInSubtree(child, aContent, aIndex);
+        }
+      }
+    } else if (content->IsXULElement(nsGkAtoms::treeseparator)) {
+      if (!content->AsElement()->AttrValueIs(kNameSpaceID_None,
+                                             nsGkAtoms::hidden,
+                                             nsGkAtoms::_true, eCaseMatters))
+        (*aIndex)++;
+    }
+  }
+}
+
+int32_t nsTreeContentView::EnsureSubtree(int32_t aIndex) {
+  Row* row = mRows[aIndex].get();
+
+  nsIContent* child;
+  child =
+      nsTreeUtils::GetImmediateChild(row->mContent, nsGkAtoms::treechildren);
+  if (!child || !child->IsXULElement()) {
+    return 0;
+  }
+
+  AutoTArray<UniquePtr<Row>, 8> rows;
+  int32_t index = 0;
+  Serialize(child, aIndex, &index, rows);
+  // Insert |rows| into |mRows| at position |aIndex|, by first creating empty
+  // UniquePtr entries and then Move'ing |rows|'s entries into them. (Note
+  // that we can't simply use InsertElementsAt with an array argument, since
+  // the destination can't steal ownership from its const source argument.)
+  UniquePtr<Row>* newRows = mRows.InsertElementsAt(aIndex + 1, rows.Length());
+  for (nsTArray<Row>::index_type i = 0; i < rows.Length(); i++) {
+    newRows[i] = std::move(rows[i]);
+  }
+  int32_t count = rows.Length();
+
+  row->mSubtreeSize += count;
+  UpdateSubtreeSizes(row->mParentIndex, count);
+
+  // Update parent indexes, but skip newly added rows.
+  // They already have correct values.
+  UpdateParentIndexes(aIndex, count + 1, count);
+
+  return count;
+}
+
+int32_t nsTreeContentView::RemoveSubtree(int32_t aIndex) {
+  Row* row = mRows[aIndex].get();
+  int32_t count = row->mSubtreeSize;
+
+  mRows.RemoveElementsAt(aIndex + 1, count);
+
+  row->mSubtreeSize -= count;
+  UpdateSubtreeSizes(row->mParentIndex, -count);
+
+  UpdateParentIndexes(aIndex, 0, -count);
+
+  return count;
+}
+
+void nsTreeContentView::InsertRowFor(nsIContent* aParent, nsIContent* aChild) {
+  int32_t grandParentIndex = -1;
+  bool insertRow = false;
+
+  nsCOMPtr<nsIContent> grandParent = aParent->GetParent();
+
+  if (grandParent->IsXULElement(nsGkAtoms::tree)) {
+    // Allow insertion to the outermost container.
+    insertRow = true;
+  } else {
+    // Test insertion to an inner container.
+
+    // First try to find this parent in our array of rows, if we find one
+    // we can be sure that all other parents are open too.
+    grandParentIndex = FindContent(grandParent);
+    if (grandParentIndex >= 0) {
+      // Got it, now test if it is open.
+      if (mRows[grandParentIndex]->IsOpen()) insertRow = true;
+    }
+  }
+
+  if (insertRow) {
+    int32_t index = 0;
+    GetIndexInSubtree(aParent, aChild, &index);
+
+    int32_t count = InsertRow(grandParentIndex, index, aChild);
+    if (mTree) mTree->RowCountChanged(grandParentIndex + index + 1, count);
+  }
+}
+
+int32_t nsTreeContentView::InsertRow(int32_t aParentIndex, int32_t aIndex,
+                                     nsIContent* aContent) {
+  AutoTArray<UniquePtr<Row>, 8> rows;
+  if (aContent->IsXULElement(nsGkAtoms::treeitem)) {
+    SerializeItem(aContent->AsElement(), aParentIndex, &aIndex, rows);
+  } else if (aContent->IsXULElement(nsGkAtoms::treeseparator)) {
+    SerializeSeparator(aContent->AsElement(), aParentIndex, &aIndex, rows);
+  }
+
+  // We can't use InsertElementsAt since the destination can't steal
+  // ownership from its const source argument.
+  int32_t count = rows.Length();
+  for (nsTArray<Row>::index_type i = 0; i < size_t(count); i++) {
+    mRows.InsertElementAt(aParentIndex + aIndex + i + 1, std::move(rows[i]));
+  }
+
+  UpdateSubtreeSizes(aParentIndex, count);
+
+  // Update parent indexes, but skip added rows.
+  // They already have correct values.
+  UpdateParentIndexes(aParentIndex + aIndex, count + 1, count);
+
+  return count;
+}
+
+int32_t nsTreeContentView::RemoveRow(int32_t aIndex) {
+  Row* row = mRows[aIndex].get();
+  int32_t count = row->mSubtreeSize + 1;
+  int32_t parentIndex = row->mParentIndex;
+
+  mRows.RemoveElementsAt(aIndex, count);
+
+  UpdateSubtreeSizes(parentIndex, -count);
+
+  UpdateParentIndexes(aIndex, 0, -count);
+
+  return count;
+}
+
+void nsTreeContentView::ClearRows() {
+  mRows.Clear();
+  mBody = nullptr;
+  // Remove ourselves from mDocument's observers.
+  if (mDocument) {
+    mDocument->RemoveObserver(this);
+    mDocument = nullptr;
+  }
+}
+
+void nsTreeContentView::OpenContainer(int32_t aIndex) {
+  Row* row = mRows[aIndex].get();
+  row->SetOpen(true);
+
+  int32_t count = EnsureSubtree(aIndex);
+  if (mTree) {
+    mTree->InvalidateRow(aIndex);
+    mTree->RowCountChanged(aIndex + 1, count);
+  }
+}
+
+void nsTreeContentView::CloseContainer(int32_t aIndex) {
+  Row* row = mRows[aIndex].get();
+  row->SetOpen(false);
+
+  int32_t count = RemoveSubtree(aIndex);
+  if (mTree) {
+    mTree->InvalidateRow(aIndex);
+    mTree->RowCountChanged(aIndex + 1, -count);
+  }
+}
+
+int32_t nsTreeContentView::FindContent(nsIContent* aContent) {
+  for (uint32_t i = 0; i < mRows.Length(); i++) {
+    if (mRows[i]->mContent == aContent) {
+      return i;
+    }
+  }
+
+  return -1;
+}
+
+void nsTreeContentView::UpdateSubtreeSizes(int32_t aParentIndex,
+                                           int32_t count) {
+  while (aParentIndex >= 0) {
+    Row* row = mRows[aParentIndex].get();
+    row->mSubtreeSize += count;
+    aParentIndex = row->mParentIndex;
+  }
+}
+
+void nsTreeContentView::UpdateParentIndexes(int32_t aIndex, int32_t aSkip,
+                                            int32_t aCount) {
+  int32_t count = mRows.Length();
+  for (int32_t i = aIndex + aSkip; i < count; i++) {
+    Row* row = mRows[i].get();
+    if (row->mParentIndex > aIndex) {
+      row->mParentIndex += aCount;
+    }
+  }
+}
+
+Element* nsTreeContentView::GetCell(nsIContent* aContainer,
+                                    nsTreeColumn& aCol) {
+  int32_t colIndex(aCol.GetIndex());
+
+  // Traverse through cells, try to find the cell by index in a row.
+  Element* result = nullptr;
+  int32_t j = 0;
+  dom::FlattenedChildIterator iter(aContainer);
+  for (nsIContent* cell = iter.GetNextChild(); cell;
+       cell = iter.GetNextChild()) {
+    if (cell->IsXULElement(nsGkAtoms::treecell)) {
+      if (j == colIndex) {
+        result = cell->AsElement();
+        break;
+      }
+      j++;
+    }
+  }
+
+  return result;
+}
+
+bool nsTreeContentView::IsValidRowIndex(int32_t aRowIndex) {
+  return aRowIndex >= 0 && aRowIndex < int32_t(mRows.Length());
+}
diff --git a/layout/xul/tree/nsTreeContentView.h b/layout/xul/tree/nsTreeContentView.h
new file mode 100644
index 0000000000..8138ab44fc
--- /dev/null
+++ b/layout/xul/tree/nsTreeContentView.h
@@ -0,0 +1,164 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeContentView_h__
+#define nsTreeContentView_h__
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsTArray.h"
+#include "nsStubDocumentObserver.h"
+#include "nsITreeView.h"
+#include "nsITreeSelection.h"
+#include "nsWrapperCache.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/UniquePtr.h"
+
+class nsSelection;
+class nsTreeColumn;
+class Row;
+
+namespace mozilla {
+class ErrorResult;
+
+namespace dom {
+class DataTransfer;
+class Document;
+class Element;
+class XULTreeElement;
+}  // namespace dom
+}  // namespace mozilla
+
+nsresult NS_NewTreeContentView(nsITreeView** aResult);
+
+class nsTreeContentView final : public nsITreeView,
+                                public nsStubDocumentObserver,
+                                public nsWrapperCache {
+  typedef mozilla::dom::Element Element;
+
+ public:
+  nsTreeContentView(void);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS_AMBIGUOUS(nsTreeContentView,
+                                                        nsITreeView)
+
+  virtual JSObject* WrapObject(JSContext* aCx,
+                               JS::Handle<JSObject*> aGivenProto) override;
+  nsISupports* GetParentObject();
+
+  int32_t RowCount() { return mRows.Length(); }
+  nsITreeSelection* GetSelection() { return mSelection; }
+  void SetSelection(nsITreeSelection* aSelection, mozilla::ErrorResult& aError);
+  void GetRowProperties(int32_t aRow, nsAString& aProperties,
+                        mozilla::ErrorResult& aError);
+  void GetCellProperties(int32_t aRow, nsTreeColumn& aColumn,
+                         nsAString& aProperies, mozilla::ErrorResult& aError);
+  void GetColumnProperties(nsTreeColumn& aColumn, nsAString& aProperies);
+  bool IsContainer(int32_t aRow, mozilla::ErrorResult& aError);
+  bool IsContainerOpen(int32_t aRow, mozilla::ErrorResult& aError);
+  bool IsContainerEmpty(int32_t aRow, mozilla::ErrorResult& aError);
+  bool IsSeparator(int32_t aRow, mozilla::ErrorResult& aError);
+  bool IsSorted() { return false; }
+  bool CanDrop(int32_t aRow, int32_t aOrientation,
+               mozilla::dom::DataTransfer* aDataTransfer,
+               mozilla::ErrorResult& aError);
+  void Drop(int32_t aRow, int32_t aOrientation,
+            mozilla::dom::DataTransfer* aDataTransfer,
+            mozilla::ErrorResult& aError);
+  int32_t GetParentIndex(int32_t aRow, mozilla::ErrorResult& aError);
+  bool HasNextSibling(int32_t aRow, int32_t aAfterIndex,
+                      mozilla::ErrorResult& aError);
+  int32_t GetLevel(int32_t aRow, mozilla::ErrorResult& aError);
+  void GetImageSrc(int32_t aRow, nsTreeColumn& aColumn, nsAString& aSrc,
+                   mozilla::ErrorResult& aError);
+  void GetCellValue(int32_t aRow, nsTreeColumn& aColumn, nsAString& aValue,
+                    mozilla::ErrorResult& aError);
+  void GetCellText(int32_t aRow, nsTreeColumn& aColumn, nsAString& aText,
+                   mozilla::ErrorResult& aError);
+  void SetTree(mozilla::dom::XULTreeElement* aTree,
+               mozilla::ErrorResult& aError);
+  void ToggleOpenState(int32_t aRow, mozilla::ErrorResult& aError);
+  void CycleHeader(nsTreeColumn& aColumn, mozilla::ErrorResult& aError);
+  void SelectionChanged() {}
+  void CycleCell(int32_t aRow, nsTreeColumn& aColumn) {}
+  bool IsEditable(int32_t aRow, nsTreeColumn& aColumn,
+                  mozilla::ErrorResult& aError);
+  void SetCellValue(int32_t aRow, nsTreeColumn& aColumn,
+                    const nsAString& aValue, mozilla::ErrorResult& aError);
+  void SetCellText(int32_t aRow, nsTreeColumn& aColumn, const nsAString& aText,
+                   mozilla::ErrorResult& aError);
+  Element* GetItemAtIndex(int32_t aRow, mozilla::ErrorResult& aError);
+  int32_t GetIndexOfItem(Element* aItem);
+
+  NS_DECL_NSITREEVIEW
+
+  // nsIDocumentObserver
+  NS_DECL_NSIMUTATIONOBSERVER_ATTRIBUTECHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTAPPENDED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
+  NS_DECL_NSIMUTATIONOBSERVER_NODEWILLBEDESTROYED
+
+  static bool CanTrustTreeSelection(nsISupports* aValue);
+
+ protected:
+  ~nsTreeContentView(void);
+
+  // Recursive methods which deal with serializing of nested content.
+  void Serialize(nsIContent* aContent, int32_t aParentIndex, int32_t* aIndex,
+                 nsTArray<mozilla::UniquePtr<Row>>& aRows);
+
+  void SerializeItem(Element* aContent, int32_t aParentIndex, int32_t* aIndex,
+                     nsTArray<mozilla::UniquePtr<Row>>& aRows);
+
+  void SerializeSeparator(Element* aContent, int32_t aParentIndex,
+                          int32_t* aIndex,
+                          nsTArray<mozilla::UniquePtr<Row>>& aRows);
+
+  void GetIndexInSubtree(nsIContent* aContainer, nsIContent* aContent,
+                         int32_t* aResult);
+
+  // Helper methods which we use to manage our plain array of rows.
+  int32_t EnsureSubtree(int32_t aIndex);
+
+  int32_t RemoveSubtree(int32_t aIndex);
+
+  int32_t InsertRow(int32_t aParentIndex, int32_t aIndex, nsIContent* aContent);
+
+  void InsertRowFor(nsIContent* aParent, nsIContent* aChild);
+
+  int32_t RemoveRow(int32_t aIndex);
+
+  void ClearRows();
+
+  void OpenContainer(int32_t aIndex);
+
+  void CloseContainer(int32_t aIndex);
+
+  int32_t FindContent(nsIContent* aContent);
+
+  void UpdateSubtreeSizes(int32_t aIndex, int32_t aCount);
+
+  void UpdateParentIndexes(int32_t aIndex, int32_t aSkip, int32_t aCount);
+
+  bool CanDrop(int32_t aRow, int32_t aOrientation,
+               mozilla::ErrorResult& aError);
+  void Drop(int32_t aRow, int32_t aOrientation, mozilla::ErrorResult& aError);
+
+  // Content helpers.
+  Element* GetCell(nsIContent* aContainer, nsTreeColumn& aCol);
+
+ private:
+  bool IsValidRowIndex(int32_t aRowIndex);
+
+  RefPtr<mozilla::dom::XULTreeElement> mTree;
+  nsCOMPtr<nsITreeSelection> mSelection;
+  nsCOMPtr<nsIContent> mBody;
+  mozilla::dom::Document* mDocument;  // WEAK
+  nsTArray<mozilla::UniquePtr<Row>> mRows;
+};
+
+#endif  // nsTreeContentView_h__
diff --git a/layout/xul/tree/nsTreeImageListener.cpp b/layout/xul/tree/nsTreeImageListener.cpp
new file mode 100644
index 0000000000..a560ada948
--- /dev/null
+++ b/layout/xul/tree/nsTreeImageListener.cpp
@@ -0,0 +1,115 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsTreeImageListener.h"
+#include "XULTreeElement.h"
+#include "imgIRequest.h"
+#include "imgIContainer.h"
+#include "nsIContent.h"
+#include "nsTreeColumns.h"
+
+using mozilla::dom::XULTreeElement;
+
+NS_IMPL_ISUPPORTS(nsTreeImageListener, imgINotificationObserver)
+
+nsTreeImageListener::nsTreeImageListener(nsTreeBodyFrame* aTreeFrame)
+    : mTreeFrame(aTreeFrame),
+      mInvalidationSuppressed(true),
+      mInvalidationArea(nullptr) {}
+
+nsTreeImageListener::~nsTreeImageListener() { delete mInvalidationArea; }
+
+void nsTreeImageListener::Notify(imgIRequest* aRequest, int32_t aType,
+                                 const nsIntRect* aData) {
+  if (aType == imgINotificationObserver::IS_ANIMATED) {
+    if (mTreeFrame) {
+      mTreeFrame->OnImageIsAnimated(aRequest);
+    }
+    return;
+  }
+
+  if (aType == imgINotificationObserver::SIZE_AVAILABLE) {
+    // Ensure the animation (if any) is started. Note: There is no
+    // corresponding call to Decrement for this. This Increment will be
+    // 'cleaned up' by the Request when it is destroyed, but only then.
+    aRequest->IncrementAnimationConsumers();
+
+    if (mTreeFrame) {
+      nsCOMPtr<imgIContainer> image;
+      aRequest->GetImage(getter_AddRefs(image));
+      if (image) {
+        nsPresContext* presContext = mTreeFrame->PresContext();
+        image->SetAnimationMode(presContext->ImageAnimationMode());
+      }
+    }
+  }
+
+  if (aType == imgINotificationObserver::FRAME_UPDATE) {
+    Invalidate();
+  }
+}
+
+void nsTreeImageListener::AddCell(int32_t aIndex, nsTreeColumn* aCol) {
+  if (!mInvalidationArea) {
+    mInvalidationArea = new InvalidationArea(aCol);
+    mInvalidationArea->AddRow(aIndex);
+  } else {
+    InvalidationArea* currArea;
+    for (currArea = mInvalidationArea; currArea;
+         currArea = currArea->GetNext()) {
+      if (currArea->GetCol() == aCol) {
+        currArea->AddRow(aIndex);
+        break;
+      }
+    }
+    if (!currArea) {
+      currArea = new InvalidationArea(aCol);
+      currArea->SetNext(mInvalidationArea);
+      mInvalidationArea = currArea;
+      mInvalidationArea->AddRow(aIndex);
+    }
+  }
+}
+
+void nsTreeImageListener::Invalidate() {
+  if (!mInvalidationSuppressed) {
+    for (InvalidationArea* currArea = mInvalidationArea; currArea;
+         currArea = currArea->GetNext()) {
+      // Loop from min to max, invalidating each cell that was listening for
+      // this image.
+      for (int32_t i = currArea->GetMin(); i <= currArea->GetMax(); ++i) {
+        if (mTreeFrame) {
+          RefPtr<XULTreeElement> tree =
+              XULTreeElement::FromNodeOrNull(mTreeFrame->GetBaseElement());
+          if (tree) {
+            tree->InvalidateCell(i, currArea->GetCol());
+          }
+        }
+      }
+    }
+  }
+}
+
+nsTreeImageListener::InvalidationArea::InvalidationArea(nsTreeColumn* aCol)
+    : mCol(aCol),
+      mMin(-1),  // min should start out "undefined"
+      mMax(0),
+      mNext(nullptr) {}
+
+void nsTreeImageListener::InvalidationArea::AddRow(int32_t aIndex) {
+  if (mMin == -1)
+    mMin = mMax = aIndex;
+  else if (aIndex < mMin)
+    mMin = aIndex;
+  else if (aIndex > mMax)
+    mMax = aIndex;
+}
+
+NS_IMETHODIMP
+nsTreeImageListener::ClearFrame() {
+  mTreeFrame = nullptr;
+  return NS_OK;
+}
diff --git a/layout/xul/tree/nsTreeImageListener.h b/layout/xul/tree/nsTreeImageListener.h
new file mode 100644
index 0000000000..f5e6e70512
--- /dev/null
+++ b/layout/xul/tree/nsTreeImageListener.h
@@ -0,0 +1,67 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeImageListener_h__
+#define nsTreeImageListener_h__
+
+#include "nsString.h"
+#include "nsCOMPtr.h"
+#include "nsTreeBodyFrame.h"
+#include "mozilla/Attributes.h"
+
+class nsTreeColumn;
+
+// This class handles image load observation.
+class nsTreeImageListener final : public imgINotificationObserver {
+ public:
+  explicit nsTreeImageListener(nsTreeBodyFrame* aTreeFrame);
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_IMGINOTIFICATIONOBSERVER
+
+  NS_IMETHOD ClearFrame();
+
+  friend class nsTreeBodyFrame;
+
+ protected:
+  ~nsTreeImageListener();
+
+  void UnsuppressInvalidation() { mInvalidationSuppressed = false; }
+  void Invalidate();
+  void AddCell(int32_t aIndex, nsTreeColumn* aCol);
+
+ private:
+  nsTreeBodyFrame* mTreeFrame;
+
+  // A guard that prevents us from recursive painting.
+  bool mInvalidationSuppressed;
+
+  class InvalidationArea {
+   public:
+    explicit InvalidationArea(nsTreeColumn* aCol);
+    ~InvalidationArea() { delete mNext; }
+
+    friend class nsTreeImageListener;
+
+   protected:
+    void AddRow(int32_t aIndex);
+    nsTreeColumn* GetCol() { return mCol.get(); }
+    int32_t GetMin() { return mMin; }
+    int32_t GetMax() { return mMax; }
+    InvalidationArea* GetNext() { return mNext; }
+    void SetNext(InvalidationArea* aNext) { mNext = aNext; }
+
+   private:
+    RefPtr<nsTreeColumn> mCol;
+    int32_t mMin;
+    int32_t mMax;
+    InvalidationArea* mNext;
+  };
+
+  InvalidationArea* mInvalidationArea;
+};
+
+#endif  // nsTreeImageListener_h__
diff --git a/layout/xul/tree/nsTreeSelection.cpp b/layout/xul/tree/nsTreeSelection.cpp
new file mode 100644
index 0000000000..952a9bf376
--- /dev/null
+++ b/layout/xul/tree/nsTreeSelection.cpp
@@ -0,0 +1,724 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/dom/Element.h"
+#include "nsCOMPtr.h"
+#include "nsTreeSelection.h"
+#include "XULTreeElement.h"
+#include "nsITreeView.h"
+#include "nsString.h"
+#include "nsIContent.h"
+#include "nsNameSpaceManager.h"
+#include "nsGkAtoms.h"
+#include "nsComponentManagerUtils.h"
+#include "nsTreeColumns.h"
+
+using namespace mozilla;
+using dom::XULTreeElement;
+
+// A helper class for managing our ranges of selection.
+struct nsTreeRange {
+  nsTreeSelection* mSelection;
+
+  nsTreeRange* mPrev;
+  nsTreeRange* mNext;
+
+  int32_t mMin;
+  int32_t mMax;
+
+  nsTreeRange(nsTreeSelection* aSel, int32_t aSingleVal)
+      : mSelection(aSel),
+        mPrev(nullptr),
+        mNext(nullptr),
+        mMin(aSingleVal),
+        mMax(aSingleVal) {}
+  nsTreeRange(nsTreeSelection* aSel, int32_t aMin, int32_t aMax)
+      : mSelection(aSel),
+        mPrev(nullptr),
+        mNext(nullptr),
+        mMin(aMin),
+        mMax(aMax) {}
+
+  ~nsTreeRange() { delete mNext; }
+
+  void Connect(nsTreeRange* aPrev = nullptr, nsTreeRange* aNext = nullptr) {
+    if (aPrev)
+      aPrev->mNext = this;
+    else
+      mSelection->mFirstRange = this;
+
+    if (aNext) aNext->mPrev = this;
+
+    mPrev = aPrev;
+    mNext = aNext;
+  }
+
+  nsresult RemoveRange(int32_t aStart, int32_t aEnd) {
+    // This should so be a loop... sigh...
+    // We start past the range to remove, so no more to remove
+    if (aEnd < mMin) return NS_OK;
+    // We are the last range to be affected
+    if (aEnd < mMax) {
+      if (aStart <= mMin) {
+        // Just chop the start of the range off
+        mMin = aEnd + 1;
+      } else {
+        // We need to split the range
+        nsTreeRange* range = new nsTreeRange(mSelection, aEnd + 1, mMax);
+        if (!range) return NS_ERROR_OUT_OF_MEMORY;
+
+        mMax = aStart - 1;
+        range->Connect(this, mNext);
+      }
+      return NS_OK;
+    }
+    nsTreeRange* next = mNext;
+    if (aStart <= mMin) {
+      // The remove includes us, remove ourselves from the list
+      if (mPrev)
+        mPrev->mNext = next;
+      else
+        mSelection->mFirstRange = next;
+
+      if (next) next->mPrev = mPrev;
+      mPrev = mNext = nullptr;
+      delete this;
+    } else if (aStart <= mMax) {
+      // Just chop the end of the range off
+      mMax = aStart - 1;
+    }
+    return next ? next->RemoveRange(aStart, aEnd) : NS_OK;
+  }
+
+  nsresult Remove(int32_t aIndex) {
+    if (aIndex >= mMin && aIndex <= mMax) {
+      // We have found the range that contains us.
+      if (mMin == mMax) {
+        // Delete the whole range.
+        if (mPrev) mPrev->mNext = mNext;
+        if (mNext) mNext->mPrev = mPrev;
+        nsTreeRange* first = mSelection->mFirstRange;
+        if (first == this) mSelection->mFirstRange = mNext;
+        mNext = mPrev = nullptr;
+        delete this;
+      } else if (aIndex == mMin)
+        mMin++;
+      else if (aIndex == mMax)
+        mMax--;
+      else {
+        // We have to break this range.
+        nsTreeRange* newRange = new nsTreeRange(mSelection, aIndex + 1, mMax);
+        if (!newRange) return NS_ERROR_OUT_OF_MEMORY;
+
+        newRange->Connect(this, mNext);
+        mMax = aIndex - 1;
+      }
+    } else if (mNext)
+      return mNext->Remove(aIndex);
+
+    return NS_OK;
+  }
+
+  nsresult Add(int32_t aIndex) {
+    if (aIndex < mMin) {
+      // We have found a spot to insert.
+      if (aIndex + 1 == mMin)
+        mMin = aIndex;
+      else if (mPrev && mPrev->mMax + 1 == aIndex)
+        mPrev->mMax = aIndex;
+      else {
+        // We have to create a new range.
+        nsTreeRange* newRange = new nsTreeRange(mSelection, aIndex);
+        if (!newRange) return NS_ERROR_OUT_OF_MEMORY;
+
+        newRange->Connect(mPrev, this);
+      }
+    } else if (mNext)
+      mNext->Add(aIndex);
+    else {
+      // Insert on to the end.
+      if (mMax + 1 == aIndex)
+        mMax = aIndex;
+      else {
+        // We have to create a new range.
+        nsTreeRange* newRange = new nsTreeRange(mSelection, aIndex);
+        if (!newRange) return NS_ERROR_OUT_OF_MEMORY;
+
+        newRange->Connect(this, nullptr);
+      }
+    }
+    return NS_OK;
+  }
+
+  bool Contains(int32_t aIndex) {
+    if (aIndex >= mMin && aIndex <= mMax) return true;
+
+    if (mNext) return mNext->Contains(aIndex);
+
+    return false;
+  }
+
+  int32_t Count() {
+    int32_t total = mMax - mMin + 1;
+    if (mNext) total += mNext->Count();
+    return total;
+  }
+
+  static void CollectRanges(nsTreeRange* aRange, nsTArray<int32_t>& aRanges) {
+    nsTreeRange* cur = aRange;
+    while (cur) {
+      aRanges.AppendElement(cur->mMin);
+      aRanges.AppendElement(cur->mMax);
+      cur = cur->mNext;
+    }
+  }
+
+  static void InvalidateRanges(XULTreeElement* aTree,
+                               nsTArray<int32_t>& aRanges) {
+    if (aTree) {
+      RefPtr<nsXULElement> tree = aTree;
+      for (uint32_t i = 0; i < aRanges.Length(); i += 2) {
+        aTree->InvalidateRange(aRanges[i], aRanges[i + 1]);
+      }
+    }
+  }
+
+  void Invalidate() {
+    nsTArray<int32_t> ranges;
+    CollectRanges(this, ranges);
+    InvalidateRanges(mSelection->mTree, ranges);
+  }
+
+  void RemoveAllBut(int32_t aIndex) {
+    if (aIndex >= mMin && aIndex <= mMax) {
+      // Invalidate everything in this list.
+      nsTArray<int32_t> ranges;
+      CollectRanges(mSelection->mFirstRange, ranges);
+
+      mMin = aIndex;
+      mMax = aIndex;
+
+      nsTreeRange* first = mSelection->mFirstRange;
+      if (mPrev) mPrev->mNext = mNext;
+      if (mNext) mNext->mPrev = mPrev;
+      mNext = mPrev = nullptr;
+
+      if (first != this) {
+        delete mSelection->mFirstRange;
+        mSelection->mFirstRange = this;
+      }
+      InvalidateRanges(mSelection->mTree, ranges);
+    } else if (mNext)
+      mNext->RemoveAllBut(aIndex);
+  }
+
+  void Insert(nsTreeRange* aRange) {
+    if (mMin >= aRange->mMax)
+      aRange->Connect(mPrev, this);
+    else if (mNext)
+      mNext->Insert(aRange);
+    else
+      aRange->Connect(this, nullptr);
+  }
+};
+
+nsTreeSelection::nsTreeSelection(XULTreeElement* aTree)
+    : mTree(aTree),
+      mSuppressed(false),
+      mCurrentIndex(-1),
+      mShiftSelectPivot(-1),
+      mFirstRange(nullptr) {}
+
+nsTreeSelection::~nsTreeSelection() {
+  delete mFirstRange;
+  if (mSelectTimer) mSelectTimer->Cancel();
+}
+
+NS_IMPL_CYCLE_COLLECTION(nsTreeSelection, mTree)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsTreeSelection)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsTreeSelection)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsTreeSelection)
+  NS_INTERFACE_MAP_ENTRY(nsITreeSelection)
+  NS_INTERFACE_MAP_ENTRY(nsINativeTreeSelection)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMETHODIMP nsTreeSelection::GetTree(XULTreeElement** aTree) {
+  NS_IF_ADDREF(*aTree = mTree);
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::SetTree(XULTreeElement* aTree) {
+  if (mSelectTimer) {
+    mSelectTimer->Cancel();
+    mSelectTimer = nullptr;
+  }
+
+  mTree = aTree;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::GetSingle(bool* aSingle) {
+  if (!mTree) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  *aSingle = mTree->AttrValueIs(kNameSpaceID_None, nsGkAtoms::seltype,
+                                u"single"_ns, eCaseMatters);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::IsSelected(int32_t aIndex, bool* aResult) {
+  if (mFirstRange)
+    *aResult = mFirstRange->Contains(aIndex);
+  else
+    *aResult = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::TimedSelect(int32_t aIndex, int32_t aMsec) {
+  bool suppressSelect = mSuppressed;
+
+  if (aMsec != -1) mSuppressed = true;
+
+  nsresult rv = Select(aIndex);
+  if (NS_FAILED(rv)) return rv;
+
+  if (aMsec != -1) {
+    mSuppressed = suppressSelect;
+    if (!mSuppressed) {
+      if (mSelectTimer) mSelectTimer->Cancel();
+
+      if (!mTree) {
+        return NS_ERROR_UNEXPECTED;
+      }
+      nsIEventTarget* target =
+          mTree->OwnerDoc()->EventTargetFor(TaskCategory::Other);
+      NS_NewTimerWithFuncCallback(getter_AddRefs(mSelectTimer), SelectCallback,
+                                  this, aMsec, nsITimer::TYPE_ONE_SHOT,
+                                  "nsTreeSelection::SelectCallback", target);
+    }
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::Select(int32_t aIndex) {
+  mShiftSelectPivot = -1;
+
+  nsresult rv = SetCurrentIndex(aIndex);
+  if (NS_FAILED(rv)) return rv;
+
+  if (mFirstRange) {
+    bool alreadySelected = mFirstRange->Contains(aIndex);
+
+    if (alreadySelected) {
+      int32_t count = mFirstRange->Count();
+      if (count > 1) {
+        // We need to deselect everything but our item.
+        mFirstRange->RemoveAllBut(aIndex);
+        FireOnSelectHandler();
+      }
+      return NS_OK;
+    } else {
+      // Clear out our selection.
+      mFirstRange->Invalidate();
+      delete mFirstRange;
+    }
+  }
+
+  // Create our new selection.
+  mFirstRange = new nsTreeRange(this, aIndex);
+  if (!mFirstRange) return NS_ERROR_OUT_OF_MEMORY;
+
+  mFirstRange->Invalidate();
+
+  // Fire the select event
+  FireOnSelectHandler();
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::ToggleSelect(int32_t aIndex) {
+  // There are six cases that can occur on a ToggleSelect with our
+  // range code.
+  // (1) A new range should be made for a selection.
+  // (2) A single range is removed from the selection.
+  // (3) The item is added to an existing range.
+  // (4) The item is removed from an existing range.
+  // (5) The addition of the item causes two ranges to be merged.
+  // (6) The removal of the item causes two ranges to be split.
+  mShiftSelectPivot = -1;
+  nsresult rv = SetCurrentIndex(aIndex);
+  if (NS_FAILED(rv)) return rv;
+
+  if (!mFirstRange)
+    Select(aIndex);
+  else {
+    if (!mFirstRange->Contains(aIndex)) {
+      bool single;
+      rv = GetSingle(&single);
+      if (NS_SUCCEEDED(rv) && !single) rv = mFirstRange->Add(aIndex);
+    } else
+      rv = mFirstRange->Remove(aIndex);
+    if (NS_SUCCEEDED(rv)) {
+      if (mTree) mTree->InvalidateRow(aIndex);
+
+      FireOnSelectHandler();
+    }
+  }
+
+  return rv;
+}
+
+NS_IMETHODIMP nsTreeSelection::RangedSelect(int32_t aStartIndex,
+                                            int32_t aEndIndex, bool aAugment) {
+  bool single;
+  nsresult rv = GetSingle(&single);
+  if (NS_FAILED(rv)) return rv;
+
+  if ((mFirstRange || (aStartIndex != aEndIndex)) && single) return NS_OK;
+
+  if (!aAugment) {
+    // Clear our selection.
+    if (mFirstRange) {
+      mFirstRange->Invalidate();
+      delete mFirstRange;
+      mFirstRange = nullptr;
+    }
+  }
+
+  if (aStartIndex == -1) {
+    if (mShiftSelectPivot != -1)
+      aStartIndex = mShiftSelectPivot;
+    else if (mCurrentIndex != -1)
+      aStartIndex = mCurrentIndex;
+    else
+      aStartIndex = aEndIndex;
+  }
+
+  mShiftSelectPivot = aStartIndex;
+  rv = SetCurrentIndex(aEndIndex);
+  if (NS_FAILED(rv)) return rv;
+
+  int32_t start = aStartIndex < aEndIndex ? aStartIndex : aEndIndex;
+  int32_t end = aStartIndex < aEndIndex ? aEndIndex : aStartIndex;
+
+  if (aAugment && mFirstRange) {
+    // We need to remove all the items within our selected range from the
+    // selection, and then we insert our new range into the list.
+    nsresult rv = mFirstRange->RemoveRange(start, end);
+    if (NS_FAILED(rv)) return rv;
+  }
+
+  nsTreeRange* range = new nsTreeRange(this, start, end);
+  if (!range) return NS_ERROR_OUT_OF_MEMORY;
+
+  range->Invalidate();
+
+  if (aAugment && mFirstRange)
+    mFirstRange->Insert(range);
+  else
+    mFirstRange = range;
+
+  FireOnSelectHandler();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::ClearRange(int32_t aStartIndex,
+                                          int32_t aEndIndex) {
+  nsresult rv = SetCurrentIndex(aEndIndex);
+  if (NS_FAILED(rv)) return rv;
+
+  if (mFirstRange) {
+    int32_t start = aStartIndex < aEndIndex ? aStartIndex : aEndIndex;
+    int32_t end = aStartIndex < aEndIndex ? aEndIndex : aStartIndex;
+
+    mFirstRange->RemoveRange(start, end);
+
+    if (mTree) mTree->InvalidateRange(start, end);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::ClearSelection() {
+  if (mFirstRange) {
+    mFirstRange->Invalidate();
+    delete mFirstRange;
+    mFirstRange = nullptr;
+  }
+  mShiftSelectPivot = -1;
+
+  FireOnSelectHandler();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::SelectAll() {
+  if (!mTree) return NS_OK;
+
+  nsCOMPtr<nsITreeView> view = mTree->GetView();
+  if (!view) return NS_OK;
+
+  int32_t rowCount;
+  view->GetRowCount(&rowCount);
+  bool single;
+  nsresult rv = GetSingle(&single);
+  if (NS_FAILED(rv)) return rv;
+
+  if (rowCount == 0 || (rowCount > 1 && single)) return NS_OK;
+
+  mShiftSelectPivot = -1;
+
+  // Invalidate not necessary when clearing selection, since
+  // we're going to invalidate the world on the SelectAll.
+  delete mFirstRange;
+
+  mFirstRange = new nsTreeRange(this, 0, rowCount - 1);
+  mFirstRange->Invalidate();
+
+  FireOnSelectHandler();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::GetRangeCount(int32_t* aResult) {
+  int32_t count = 0;
+  nsTreeRange* curr = mFirstRange;
+  while (curr) {
+    count++;
+    curr = curr->mNext;
+  }
+
+  *aResult = count;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::GetRangeAt(int32_t aIndex, int32_t* aMin,
+                                          int32_t* aMax) {
+  *aMin = *aMax = -1;
+  int32_t i = -1;
+  nsTreeRange* curr = mFirstRange;
+  while (curr) {
+    i++;
+    if (i == aIndex) {
+      *aMin = curr->mMin;
+      *aMax = curr->mMax;
+      break;
+    }
+    curr = curr->mNext;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::GetCount(int32_t* count) {
+  if (mFirstRange)
+    *count = mFirstRange->Count();
+  else  // No range available, so there's no selected row.
+    *count = 0;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::GetSelectEventsSuppressed(
+    bool* aSelectEventsSuppressed) {
+  *aSelectEventsSuppressed = mSuppressed;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::SetSelectEventsSuppressed(
+    bool aSelectEventsSuppressed) {
+  mSuppressed = aSelectEventsSuppressed;
+  if (!mSuppressed) FireOnSelectHandler();
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::GetCurrentIndex(int32_t* aCurrentIndex) {
+  *aCurrentIndex = mCurrentIndex;
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsTreeSelection::SetCurrentIndex(int32_t aIndex) {
+  if (!mTree) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  if (mCurrentIndex == aIndex) {
+    return NS_OK;
+  }
+  if (mCurrentIndex != -1 && mTree) mTree->InvalidateRow(mCurrentIndex);
+
+  mCurrentIndex = aIndex;
+  if (!mTree) return NS_OK;
+
+  if (aIndex != -1) mTree->InvalidateRow(aIndex);
+
+  // Fire DOMMenuItemActive or DOMMenuItemInactive event for tree.
+  NS_ENSURE_STATE(mTree);
+
+  constexpr auto DOMMenuItemActive = u"DOMMenuItemActive"_ns;
+  constexpr auto DOMMenuItemInactive = u"DOMMenuItemInactive"_ns;
+
+  RefPtr<AsyncEventDispatcher> asyncDispatcher = new AsyncEventDispatcher(
+      mTree, (aIndex != -1 ? DOMMenuItemActive : DOMMenuItemInactive),
+      CanBubble::eYes, ChromeOnlyDispatch::eNo);
+  return asyncDispatcher->PostDOMEvent();
+}
+
+#define ADD_NEW_RANGE(macro_range, macro_selection, macro_start, macro_end) \
+  {                                                                         \
+    int32_t start = macro_start;                                            \
+    int32_t end = macro_end;                                                \
+    if (start > end) {                                                      \
+      end = start;                                                          \
+    }                                                                       \
+    nsTreeRange* macro_new_range =                                          \
+        new nsTreeRange(macro_selection, start, end);                       \
+    if (macro_range)                                                        \
+      macro_range->Insert(macro_new_range);                                 \
+    else                                                                    \
+      macro_range = macro_new_range;                                        \
+  }
+
+NS_IMETHODIMP
+nsTreeSelection::AdjustSelection(int32_t aIndex, int32_t aCount) {
+  NS_ASSERTION(aCount != 0, "adjusting by zero");
+  if (!aCount) return NS_OK;
+
+  // adjust mShiftSelectPivot, if necessary
+  if ((mShiftSelectPivot != 1) && (aIndex <= mShiftSelectPivot)) {
+    // if we are deleting and the delete includes the shift select pivot, reset
+    // it
+    if (aCount < 0 && (mShiftSelectPivot <= (aIndex - aCount - 1))) {
+      mShiftSelectPivot = -1;
+    } else {
+      mShiftSelectPivot += aCount;
+    }
+  }
+
+  // adjust mCurrentIndex, if necessary
+  if ((mCurrentIndex != -1) && (aIndex <= mCurrentIndex)) {
+    // if we are deleting and the delete includes the current index, reset it
+    if (aCount < 0 && (mCurrentIndex <= (aIndex - aCount - 1))) {
+      mCurrentIndex = -1;
+    } else {
+      mCurrentIndex += aCount;
+    }
+  }
+
+  // no selection, so nothing to do.
+  if (!mFirstRange) return NS_OK;
+
+  bool selChanged = false;
+  nsTreeRange* oldFirstRange = mFirstRange;
+  nsTreeRange* curr = mFirstRange;
+  mFirstRange = nullptr;
+  while (curr) {
+    if (aCount > 0) {
+      // inserting
+      if (aIndex > curr->mMax) {
+        // adjustment happens after the range, so no change
+        ADD_NEW_RANGE(mFirstRange, this, curr->mMin, curr->mMax);
+      } else if (aIndex <= curr->mMin) {
+        // adjustment happens before the start of the range, so shift down
+        ADD_NEW_RANGE(mFirstRange, this, curr->mMin + aCount,
+                      curr->mMax + aCount);
+        selChanged = true;
+      } else {
+        // adjustment happen inside the range.
+        // break apart the range and create two ranges
+        ADD_NEW_RANGE(mFirstRange, this, curr->mMin, aIndex - 1);
+        ADD_NEW_RANGE(mFirstRange, this, aIndex + aCount, curr->mMax + aCount);
+        selChanged = true;
+      }
+    } else {
+      // deleting
+      if (aIndex > curr->mMax) {
+        // adjustment happens after the range, so no change
+        ADD_NEW_RANGE(mFirstRange, this, curr->mMin, curr->mMax);
+      } else {
+        // remember, aCount is negative
+        selChanged = true;
+        int32_t lastIndexOfAdjustment = aIndex - aCount - 1;
+        if (aIndex <= curr->mMin) {
+          if (lastIndexOfAdjustment < curr->mMin) {
+            // adjustment happens before the start of the range, so shift up
+            ADD_NEW_RANGE(mFirstRange, this, curr->mMin + aCount,
+                          curr->mMax + aCount);
+          } else if (lastIndexOfAdjustment >= curr->mMax) {
+            // adjustment contains the range.  remove the range by not adding it
+            // to the newRange
+          } else {
+            // adjustment starts before the range, and ends in the middle of it,
+            // so trim the range
+            ADD_NEW_RANGE(mFirstRange, this, aIndex, curr->mMax + aCount)
+          }
+        } else if (lastIndexOfAdjustment >= curr->mMax) {
+          // adjustment starts in the middle of the current range, and contains
+          // the end of the range, so trim the range
+          ADD_NEW_RANGE(mFirstRange, this, curr->mMin, aIndex - 1)
+        } else {
+          // range contains the adjustment, so shorten the range
+          ADD_NEW_RANGE(mFirstRange, this, curr->mMin, curr->mMax + aCount)
+        }
+      }
+    }
+    curr = curr->mNext;
+  }
+
+  delete oldFirstRange;
+
+  // Fire the select event
+  if (selChanged) FireOnSelectHandler();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTreeSelection::InvalidateSelection() {
+  if (mFirstRange) mFirstRange->Invalidate();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTreeSelection::GetShiftSelectPivot(int32_t* aIndex) {
+  *aIndex = mShiftSelectPivot;
+  return NS_OK;
+}
+
+nsresult nsTreeSelection::FireOnSelectHandler() {
+  if (mSuppressed || !mTree) {
+    return NS_OK;
+  }
+
+  AsyncEventDispatcher::RunDOMEventWhenSafe(
+      *mTree, u"select"_ns, CanBubble::eYes, ChromeOnlyDispatch::eNo);
+  return NS_OK;
+}
+
+void nsTreeSelection::SelectCallback(nsITimer* aTimer, void* aClosure) {
+  RefPtr<nsTreeSelection> self = static_cast<nsTreeSelection*>(aClosure);
+  if (self) {
+    self->FireOnSelectHandler();
+    aTimer->Cancel();
+    self->mSelectTimer = nullptr;
+  }
+}
+
+///////////////////////////////////////////////////////////////////////////////////
+
+nsresult NS_NewTreeSelection(XULTreeElement* aTree,
+                             nsITreeSelection** aResult) {
+  *aResult = new nsTreeSelection(aTree);
+  if (!*aResult) return NS_ERROR_OUT_OF_MEMORY;
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
diff --git a/layout/xul/tree/nsTreeSelection.h b/layout/xul/tree/nsTreeSelection.h
new file mode 100644
index 0000000000..3b0eb6b21e
--- /dev/null
+++ b/layout/xul/tree/nsTreeSelection.h
@@ -0,0 +1,56 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeSelection_h__
+#define nsTreeSelection_h__
+
+#include "nsITreeSelection.h"
+#include "nsITimer.h"
+#include "nsCycleCollectionParticipant.h"
+#include "mozilla/Attributes.h"
+#include "XULTreeElement.h"
+
+class nsTreeColumn;
+struct nsTreeRange;
+
+class nsTreeSelection final : public nsINativeTreeSelection {
+ public:
+  explicit nsTreeSelection(mozilla::dom::XULTreeElement* aTree);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(nsTreeSelection)
+  NS_DECL_NSITREESELECTION
+
+  // nsINativeTreeSelection: Untrusted code can use us
+  NS_IMETHOD EnsureNative() override { return NS_OK; }
+
+  friend struct nsTreeRange;
+
+ protected:
+  ~nsTreeSelection();
+
+  nsresult FireOnSelectHandler();
+  static void SelectCallback(nsITimer* aTimer, void* aClosure);
+
+ protected:
+  // The tree will hold on to us through the view and let go when it dies.
+  RefPtr<mozilla::dom::XULTreeElement> mTree;
+
+  bool mSuppressed;       // Whether or not we should be firing onselect events.
+  int32_t mCurrentIndex;  // The item to draw the rect around. The last one
+                          // clicked, etc.
+  int32_t mShiftSelectPivot;  // Used when multiple SHIFT+selects are performed
+                              // to pivot on.
+
+  nsTreeRange* mFirstRange;  // Our list of ranges.
+
+  nsCOMPtr<nsITimer> mSelectTimer;
+};
+
+nsresult NS_NewTreeSelection(mozilla::dom::XULTreeElement* aTree,
+                             nsITreeSelection** aResult);
+
+#endif
diff --git a/layout/xul/tree/nsTreeStyleCache.cpp b/layout/xul/tree/nsTreeStyleCache.cpp
new file mode 100644
index 0000000000..d5823ac38e
--- /dev/null
+++ b/layout/xul/tree/nsTreeStyleCache.cpp
@@ -0,0 +1,103 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsTreeStyleCache.h"
+#include "mozilla/ComputedStyleInlines.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/ServoStyleSet.h"
+#include "nsPresContextInlines.h"
+
+using namespace mozilla;
+
+nsTreeStyleCache::Transition::Transition(DFAState aState, nsAtom* aSymbol)
+    : mState(aState), mInputSymbol(aSymbol) {}
+
+bool nsTreeStyleCache::Transition::operator==(const Transition& aOther) const {
+  return aOther.mState == mState && aOther.mInputSymbol == mInputSymbol;
+}
+
+uint32_t nsTreeStyleCache::Transition::Hash() const {
+  // Make a 32-bit integer that combines the low-order 16 bits of the state and
+  // the input symbol.
+  uint32_t hb = mState << 16;
+  uint32_t lb = (NS_PTR_TO_UINT32(mInputSymbol.get()) << 16) >> 16;
+  return hb + lb;
+}
+
+// The ComputedStyle cache impl
+ComputedStyle* nsTreeStyleCache::GetComputedStyle(
+    nsPresContext* aPresContext, nsIContent* aContent, ComputedStyle* aStyle,
+    nsCSSAnonBoxPseudoStaticAtom* aPseudoElement, const AtomArray& aInputWord) {
+  MOZ_ASSERT(nsCSSAnonBoxes::IsTreePseudoElement(aPseudoElement));
+
+  uint32_t count = aInputWord.Length();
+
+  // Go ahead and init the transition table.
+  if (!mTransitionTable) {
+    // Automatic miss. Build the table
+    mTransitionTable = MakeUnique<TransitionTable>();
+  }
+
+  // The first transition is always made off the supplied pseudo-element.
+  Transition transition(0, aPseudoElement);
+  DFAState currState = mTransitionTable->Get(transition);
+
+  if (!currState) {
+    // We had a miss. Make a new state and add it to our hash.
+    currState = mNextState;
+    mNextState++;
+    mTransitionTable->InsertOrUpdate(transition, currState);
+  }
+
+  for (uint32_t i = 0; i < count; i++) {
+    Transition transition(currState, aInputWord[i]);
+    currState = mTransitionTable->Get(transition);
+
+    if (!currState) {
+      // We had a miss. Make a new state and add it to our hash.
+      currState = mNextState;
+      mNextState++;
+      mTransitionTable->InsertOrUpdate(transition, currState);
+    }
+  }
+
+  // We're in a final state.
+  // Look up our ComputedStyle for this state.
+  ComputedStyle* result = nullptr;
+  if (mCache) {
+    result = mCache->GetWeak(currState);
+  }
+  if (!result) {
+    // We missed the cache. Resolve this pseudo-style.
+    RefPtr<ComputedStyle> newResult =
+        aPresContext->StyleSet()->ResolveXULTreePseudoStyle(
+            aContent->AsElement(), aPseudoElement, aStyle, aInputWord);
+
+    // Normally we rely on nsIFrame::Init / RestyleManager to call this, but
+    // these are weird and don't use a frame, yet ::-moz-tree-twisty definitely
+    // pokes at list-style-image.
+    newResult->StartImageLoads(*aPresContext->Document());
+
+    // Even though xul-tree pseudos are defined in nsCSSAnonBoxList, nothing has
+    // ever treated them as an anon box, and they don't ever get boxes anyway.
+    //
+    // This is really weird, and probably nothing really relies on the result of
+    // these assert, but it's here just to avoid changing them accidentally.
+    MOZ_ASSERT(newResult->GetPseudoType() == PseudoStyleType::XULTree);
+    MOZ_ASSERT(!newResult->IsAnonBox());
+    MOZ_ASSERT(!newResult->IsPseudoElement());
+
+    // Put the ComputedStyle in our table, transferring the owning reference to
+    // the table.
+    if (!mCache) {
+      mCache = MakeUnique<ComputedStyleCache>();
+    }
+    result = newResult.get();
+    mCache->InsertOrUpdate(currState, std::move(newResult));
+  }
+
+  return result;
+}
diff --git a/layout/xul/tree/nsTreeStyleCache.h b/layout/xul/tree/nsTreeStyleCache.h
new file mode 100644
index 0000000000..b0c0e4d9dc
--- /dev/null
+++ b/layout/xul/tree/nsTreeStyleCache.h
@@ -0,0 +1,82 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeStyleCache_h__
+#define nsTreeStyleCache_h__
+
+#include "mozilla/AtomArray.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/UniquePtr.h"
+#include "nsCOMArray.h"
+#include "nsTHashMap.h"
+#include "nsRefPtrHashtable.h"
+#include "mozilla/ComputedStyle.h"
+
+class nsIContent;
+
+class nsTreeStyleCache {
+ public:
+  nsTreeStyleCache() : mNextState(0) {}
+
+  ~nsTreeStyleCache() { Clear(); }
+
+  void Clear() {
+    mTransitionTable = nullptr;
+    mCache = nullptr;
+    mNextState = 0;
+  }
+
+  mozilla::ComputedStyle* GetComputedStyle(
+      nsPresContext* aPresContext, nsIContent* aContent,
+      mozilla::ComputedStyle* aStyle,
+      nsCSSAnonBoxPseudoStaticAtom* aPseudoElement,
+      const mozilla::AtomArray& aInputWord);
+
+ protected:
+  typedef uint32_t DFAState;
+
+  class Transition final {
+   public:
+    Transition(DFAState aState, nsAtom* aSymbol);
+    bool operator==(const Transition& aOther) const;
+    uint32_t Hash() const;
+
+   private:
+    DFAState mState;
+    RefPtr<nsAtom> mInputSymbol;
+  };
+
+  typedef nsTHashMap<nsGenericHashKey<Transition>, DFAState> TransitionTable;
+
+  // A transition table for a deterministic finite automaton.  The DFA
+  // takes as its input a single pseudoelement and an ordered set of properties.
+  // It transitions on an input word that is the concatenation of the
+  // pseudoelement supplied with the properties in the array.
+  //
+  // It transitions from state to state by looking up entries in the transition
+  // table (which is a mapping from (S,i)->S', where S is the current state, i
+  // is the next property in the input word, and S' is the state to transition
+  // to.
+  //
+  // If S' is not found, it is constructed and entered into the hashtable
+  // under the key (S,i).
+  //
+  // Once the entire word has been consumed, the final state is used
+  // to reference the cache table to locate the ComputedStyle.
+  mozilla::UniquePtr<TransitionTable> mTransitionTable;
+
+  // The cache of all active ComputedStyles.  This is a hash from
+  // a final state in the DFA, Sf, to the resultant ComputedStyle.
+  typedef nsRefPtrHashtable<nsUint32HashKey, mozilla::ComputedStyle>
+      ComputedStyleCache;
+  mozilla::UniquePtr<ComputedStyleCache> mCache;
+
+  // An integer counter that is used when we need to make new states in the
+  // DFA.
+  DFAState mNextState;
+};
+
+#endif  // nsTreeStyleCache_h__
diff --git a/layout/xul/tree/nsTreeUtils.cpp b/layout/xul/tree/nsTreeUtils.cpp
new file mode 100644
index 0000000000..10767e22d2
--- /dev/null
+++ b/layout/xul/tree/nsTreeUtils.cpp
@@ -0,0 +1,135 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsReadableUtils.h"
+#include "nsTreeUtils.h"
+#include "ChildIterator.h"
+#include "nsCRT.h"
+#include "nsAtom.h"
+#include "nsNameSpaceManager.h"
+#include "nsGkAtoms.h"
+#include "nsIContent.h"
+
+using namespace mozilla;
+
+nsresult nsTreeUtils::TokenizeProperties(const nsAString& aProperties,
+                                         AtomArray& aPropertiesArray) {
+  nsAString::const_iterator end;
+  aProperties.EndReading(end);
+
+  nsAString::const_iterator iter;
+  aProperties.BeginReading(iter);
+
+  do {
+    // Skip whitespace
+    while (iter != end && nsCRT::IsAsciiSpace(*iter)) ++iter;
+
+    // If only whitespace, we're done
+    if (iter == end) break;
+
+    // Note the first non-whitespace character
+    nsAString::const_iterator first = iter;
+
+    // Advance to the next whitespace character
+    while (iter != end && !nsCRT::IsAsciiSpace(*iter)) ++iter;
+
+    // XXX this would be nonsensical
+    NS_ASSERTION(iter != first, "eh? something's wrong here");
+    if (iter == first) break;
+
+    RefPtr<nsAtom> atom = NS_Atomize(Substring(first, iter));
+    aPropertiesArray.AppendElement(atom);
+  } while (iter != end);
+
+  return NS_OK;
+}
+
+nsIContent* nsTreeUtils::GetImmediateChild(nsIContent* aContainer,
+                                           nsAtom* aTag) {
+  dom::FlattenedChildIterator iter(aContainer);
+  for (nsIContent* child = iter.GetNextChild(); child;
+       child = iter.GetNextChild()) {
+    if (child->IsXULElement(aTag)) {
+      return child;
+    }
+    // <slot> is in the flattened tree, but <tree> code is used to work with
+    // <xbl:children> which is not, so recurse in <slot> here.
+    if (child->IsHTMLElement(nsGkAtoms::slot)) {
+      if (nsIContent* c = GetImmediateChild(child, aTag)) {
+        return c;
+      }
+    }
+  }
+
+  return nullptr;
+}
+
+nsIContent* nsTreeUtils::GetDescendantChild(nsIContent* aContainer,
+                                            nsAtom* aTag) {
+  dom::FlattenedChildIterator iter(aContainer);
+  for (nsIContent* child = iter.GetNextChild(); child;
+       child = iter.GetNextChild()) {
+    if (child->IsXULElement(aTag)) {
+      return child;
+    }
+
+    child = GetDescendantChild(child, aTag);
+    if (child) {
+      return child;
+    }
+  }
+
+  return nullptr;
+}
+
+nsresult nsTreeUtils::UpdateSortIndicators(dom::Element* aColumn,
+                                           const nsAString& aDirection) {
+  aColumn->SetAttr(kNameSpaceID_None, nsGkAtoms::sortDirection, aDirection,
+                   true);
+  aColumn->SetAttr(kNameSpaceID_None, nsGkAtoms::sortActive, u"true"_ns, true);
+
+  // Unset sort attribute(s) on the other columns
+  nsCOMPtr<nsIContent> parentContent = aColumn->GetParent();
+  if (parentContent && parentContent->NodeInfo()->Equals(nsGkAtoms::treecols,
+                                                         kNameSpaceID_XUL)) {
+    for (nsINode* childContent = parentContent->GetFirstChild(); childContent;
+         childContent = childContent->GetNextSibling()) {
+      if (childContent != aColumn &&
+          childContent->NodeInfo()->Equals(nsGkAtoms::treecol,
+                                           kNameSpaceID_XUL)) {
+        childContent->AsElement()->UnsetAttr(kNameSpaceID_None,
+                                             nsGkAtoms::sortDirection, true);
+        childContent->AsElement()->UnsetAttr(kNameSpaceID_None,
+                                             nsGkAtoms::sortActive, true);
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+nsresult nsTreeUtils::GetColumnIndex(dom::Element* aColumn, int32_t* aResult) {
+  nsIContent* parentContent = aColumn->GetParent();
+  if (parentContent && parentContent->NodeInfo()->Equals(nsGkAtoms::treecols,
+                                                         kNameSpaceID_XUL)) {
+    int32_t colIndex = 0;
+
+    for (nsINode* childContent = parentContent->GetFirstChild(); childContent;
+         childContent = childContent->GetNextSibling()) {
+      if (childContent->NodeInfo()->Equals(nsGkAtoms::treecol,
+                                           kNameSpaceID_XUL)) {
+        if (childContent == aColumn) {
+          *aResult = colIndex;
+          return NS_OK;
+        }
+        ++colIndex;
+      }
+    }
+  }
+
+  *aResult = -1;
+  return NS_OK;
+}
diff --git a/layout/xul/tree/nsTreeUtils.h b/layout/xul/tree/nsTreeUtils.h
new file mode 100644
index 0000000000..d0588f1273
--- /dev/null
+++ b/layout/xul/tree/nsTreeUtils.h
@@ -0,0 +1,43 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTreeUtils_h__
+#define nsTreeUtils_h__
+
+#include "mozilla/AtomArray.h"
+#include "nsError.h"
+#include "nsString.h"
+#include "nsTreeStyleCache.h"
+
+class nsAtom;
+class nsIContent;
+namespace mozilla {
+namespace dom {
+class Element;
+}
+}  // namespace mozilla
+
+class nsTreeUtils {
+ public:
+  /**
+   * Parse a whitespace separated list of properties into an array
+   * of atoms.
+   */
+  static nsresult TokenizeProperties(const nsAString& aProperties,
+                                     mozilla::AtomArray& aPropertiesArray);
+
+  static nsIContent* GetImmediateChild(nsIContent* aContainer, nsAtom* aTag);
+
+  static nsIContent* GetDescendantChild(nsIContent* aContainer, nsAtom* aTag);
+
+  static nsresult UpdateSortIndicators(mozilla::dom::Element* aColumn,
+                                       const nsAString& aDirection);
+
+  static nsresult GetColumnIndex(mozilla::dom::Element* aColumn,
+                                 int32_t* aResult);
+};
+
+#endif  // nsTreeUtils_h__