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diff --git a/layout/reftests/invalidation/layer-splitting-5.html b/layout/reftests/invalidation/layer-splitting-5.html
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+<!DOCTYPE html>
+<html lang="en" class="reftest-wait"
+      reftest-displayport-x="0"
+      reftest-displayport-y="0"
+      reftest-displayport-w="800"
+      reftest-displayport-h="1000">
+<meta charset="utf-8">
+<title>Things overlapping active scrollboxes should be in a layer on top of the scrolled contents.</title>
+
+<style>
+
+div {
+  height: 50px;
+  border: 1px solid;
+  box-model: border-box;
+}
+
+.first, .second {
+  border-color: blue;
+  margin: 50px 0;
+}
+
+.overlap {
+  border-color: #088;
+  margin-left: 100px;
+  width: 80px;
+  margin-bottom: -30px;
+  position: relative;
+}
+
+.scrollable {
+  height: auto;
+  overflow: auto;
+}
+
+.scrollarea {
+  width: 5000px;
+  border: none;
+  padding: 10px 0 20px;
+  height: auto;
+}
+
+.scrolled {
+  margin-left: 220px;
+  width: 100px;
+  height: 100px;
+  border-color: red;
+}
+
+body {
+  margin: 0;
+  padding: 0 100px;
+  height: 3000px;
+}
+
+</style>
+
+<div class="first" reftest-assigned-layer="page-background">
+  <!--
+    This is just a regular box, it should end up in the page background layer.
+  -->
+</div>
+
+<div class="overlap reftest-no-paint">
+  <!--
+    This item intersects with the scrollable box and is positioned above
+    .scrolled, in z-order, so it should be split into its own layer as soon
+    as the scrollbox gets active scrolling. The splitting should not wait for
+    .scrolled to move under .overlap.
+  -->
+</div>
+
+<div class="scrollable">
+  <div class="scrollarea">
+    <div class="scrolled reftest-opaque-layer">
+      <!--
+        This will move under .overlap by .scrollable being scrolled. This
+        action should not invalidate .overlap.
+
+        Furthermore, since the background of .scrollable is uniform and opaque,
+        .scrolled should be able to pull up that background color and become
+        opaque itself.
+      -->
+    </div>
+  </div>
+</div>
+
+<div class="second" reftest-assigned-layer="page-background">
+  <!--
+    This should share a layer with .first and the page background.
+  -->
+</div>
+
+<script>
+
+var scrollable = document.querySelector(".scrollable");
+
+function doTest() {
+  scrollable.scrollLeft = 100;
+  document.documentElement.removeAttribute("class");
+}
+
+// Make .scrollable start out with active scrolling.
+scrollable.scrollLeft = 0;
+scrollable.scrollLeft = 20;
+document.addEventListener("MozReftestInvalidate", doTest);
+
+</script>
+