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+<!DOCTYPE HTML>
+<html class="reftest-wait">
+<head>
+    <meta charset="utf-8">
+    <title>Testcase for bug 1010538 - Dynamically change scroll-behavior</title>
+    <style type="text/css">
+
+        html,body {
+            color: black;
+            background-color: white;
+            font-size: 16px;
+            padding: 0;
+            margin: 0;
+        }
+
+        .a_box {
+            position: relative;
+            left: 0px;
+            top: 0px;
+            width: 20px;
+            height: 20px;
+            background: blue;
+        }
+
+        .another_box {
+            position: relative;
+            left: 2000px;
+            top: 2000px;
+            width: 20px;
+            height: 20px;
+            background: green;
+        }
+
+        .scroll_box {
+            width: 150px;
+            height: 150px;
+            overflow: scroll;
+        }
+
+        #scroll_2 {
+            scroll-behavior: auto;
+        }
+
+        #scroll_3 {
+            scroll-behavior: smooth;
+        }
+
+    </style>
+</head>
+<body>
+  <div id="scroll_1" class="scroll_box">
+    <div id="box1a" class="a_box"></div>
+    <div id="box1b" class="another_box"></div>
+  </div>
+  <div id="scroll_2" class="scroll_box">
+    <div id="box2a" class="a_box"></div>
+    <div id="box2b" class="another_box"></div>
+  </div>
+  <div id="scroll_3" class="scroll_box">
+    <div id="box3a" class="a_box"></div>
+    <div id="box3b" class="another_box"></div>
+  </div>
+<script>
+  function doTest() {
+    if (document.location.search != '?ref') {
+      // Expect smooth scroll:
+      document.getElementById("scroll_1").style.scrollBehavior = "smooth";
+      document.getElementById("scroll_1").scrollTo({left: 0, top: 0});
+      document.getElementById("scroll_2").style.scrollBehavior = "smooth";
+      document.getElementById("scroll_2").scrollTo({left: 0, top: 0});
+
+      // Expect instant scroll:
+      document.getElementById("scroll_3").style.scrollBehavior = "auto";
+      document.getElementById("scroll_3").scrollTo({left: 0, top: 0});
+    } else {
+      document.getElementById("scroll_1").scrollTo({left: 0, top: 0, behavior: "smooth"});
+      document.getElementById("scroll_2").scrollTo({left: 0, top: 0, behavior: "smooth"});
+      document.getElementById("scroll_3").scrollTo({left: 0, top: 0, behavior: "instant"});
+    }
+
+    // Interrupt any smooth scrolling
+    for (var i=1; i <= 3; i++) {
+      document.getElementById("scroll_" + i).scrollTo();
+    }
+
+    document.documentElement.removeAttribute("class");
+  }
+
+  for (var i=1; i <= 3; i++) {
+    document.getElementById("box" + i + "b")
+      .scrollIntoView({block: "start", behavior: "instant"});
+  }
+
+  window.addEventListener("MozReftestInvalidate", doTest);
+</script>
+</body>
+</html>