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+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width; initial-scale=1.0">
+  <title>Dragging the mouse on a transformed scrollframe inside a fixed-pos element</title>
+  <script type="application/javascript" src="apz_test_native_event_utils.js"></script>
+  <script type="application/javascript" src="apz_test_utils.js"></script>
+  <script src="/tests/SimpleTest/paint_listener.js"></script>
+  <script type="text/javascript">
+
+async function test() {
+  var scrollableDiv = document.getElementById("scrollable");
+  let scrollPromise = new Promise(resolve => {
+    scrollableDiv.addEventListener("scroll", resolve, {once: true});
+  });
+
+  // Scroll down a small amount (10px). The bug in this case is that the
+  // scrollthumb remains a little "above" where it's supposed to be, so if the
+  // bug manifests here, then the thumb will remain at the top of the track
+  // and the scroll position will remain at 0.
+  var dragFinisher = await promiseVerticalScrollbarDrag(scrollableDiv, 10, 10);
+  if (!dragFinisher) {
+    ok(true, "No scrollbar, can't do this test");
+    return;
+  }
+
+  // the events above might be stuck in APZ input queue for a bit until the
+  // layer is activated, so we wait here until the scroll event listener is
+  // triggered.
+  await scrollPromise;
+
+  await dragFinisher();
+
+  // Flush everything just to be safe
+  await promiseOnlyApzControllerFlushed();
+
+  // In this case we just want to make sure the scroll position moved from 0
+  // which indicates the thumb dragging worked properly.
+  ok(scrollableDiv.scrollTop > 0, "Scrollbar drag resulted in a scroll position of " + scrollableDiv.scrollTop);
+}
+
+waitUntilApzStable()
+.then(test)
+.then(subtestDone, subtestFailed);
+
+  </script>
+  <style>
+    #fixed {
+      position: fixed;
+      left: 0;
+      top: 0;
+      width: 300px;
+      height: 100%;
+    }
+    #scrollable {
+      transform: translateY(100px);
+      overflow: scroll;
+      height: 100%;
+    }
+    #content {
+      height: 5000px;
+      background-image: linear-gradient(red,blue);
+    }
+  </style>
+</head>
+<body>
+<div id="fixed">
+  <div id="scrollable">
+    <div id="content"></div>
+  </div>
+</div>
+</body>
+</html>