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+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width">
+  <title>Clicking on the scrollbar track in quick succession should scroll the right amount</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 src="/tests/SimpleTest/EventUtils.js"></script>
+
+  <script type="application/javascript">
+
+// A helper to synthesize a native mouse click on a scrollbar track,
+// and wait long enough such that subsequent ticking of the refresh
+// driver will progress any resulting scroll animation.
+// In particular, just `await promiseNativeMouseEventWithApz(...)`
+// is not enough, it waits for the synthesization messages to arrive
+// in the parent process, but the native events may still be in the
+// OS event queue. Instead, we need to wait for a synthesized event
+// to arrive at content. While we're synthesizing a "click", if the
+// target is a scrollbar the window only gets the "mousedown" and
+// "mouseup", not a "click". Waiting for the "mousedown" is not enough
+// (the "mouseup" can still be stuck in the event queue), so we wait
+// for "mouseup".
+async function promiseNativeMouseClickOnScrollbarTrack(anchor, xOffset, yOffset) {
+  await promiseNativeMouseEventWithAPZAndWaitForEvent({
+    type: "click",
+    target: anchor,
+    offsetX: xOffset,
+    offsetY: yOffset,
+    eventTypeToWait: "mouseup"
+  });
+}
+
+async function test() {
+  var scroller = document.documentElement;
+  var verticalScrollbarWidth = window.innerWidth - scroller.clientWidth;
+
+  if (verticalScrollbarWidth == 0) {
+    ok(true, "Scrollbar width is zero on this platform, test is useless here");
+    return;
+  }
+
+  // The anchor is the fixed-pos div that we use to calculate coordinates to
+  // click on the scrollbar. That way we don't have to recompute coordinates
+  // as the page scrolls. The anchor is at the bottom-right corner of the
+  // content area.
+  var anchor = document.getElementById('anchor');
+
+  var xoffset = (verticalScrollbarWidth / 2);
+  // Get a y-coord near the bottom of the vertical scrollbar track. Assume the
+  // vertical thumb is near the top of the scrollback track (since scroll
+  // position starts off at zero) and won't get in the way. Also assume the
+  // down arrow button, if there is one, is square.
+  var yoffset = 0 - verticalScrollbarWidth - 5;
+
+  // Take control of the refresh driver
+  let utils = SpecialPowers.getDOMWindowUtils(window);
+  utils.advanceTimeAndRefresh(0);
+
+  // Click at the bottom of the scrollbar track to trigger a page-down kind of
+  // scroll. This should use "desktop zooming" scrollbar code which should
+  // trigger an APZ scroll animation.
+  await promiseNativeMouseClickOnScrollbarTrack(anchor, xoffset, yoffset);
+
+  // Run 1000 frames, that should be enough to let the scroll animation start
+  // and run to completion. We check that it scrolled at least half the visible
+  // height, since we expect about a full screen height minus a few lines.
+  for (let i = 0; i < 1000; i++) {
+    utils.advanceTimeAndRefresh(16);
+  }
+  await promiseOnlyApzControllerFlushed();
+
+  let pageScrollAmount = scroller.scrollTop;
+  ok(pageScrollAmount > scroller.clientHeight / 2,
+     `Scroll offset is ${pageScrollAmount}, should be near clientHeight ${scroller.clientHeight}`);
+
+  // Now we do two clicks in quick succession, but with a few frames in between
+  // to verify the scroll animation from the first click is active before the
+  // second click happens.
+  await promiseNativeMouseClickOnScrollbarTrack(anchor, xoffset, yoffset);
+  for (let i = 0; i < 5; i++) {
+    utils.advanceTimeAndRefresh(16);
+  }
+  await promiseOnlyApzControllerFlushed();
+  let curPos = scroller.scrollTop;
+  ok(curPos > pageScrollAmount, `Scroll offset has increased to ${curPos}`);
+  ok(curPos < pageScrollAmount * 2, "Second page-scroll is not yet complete");
+  await promiseNativeMouseClickOnScrollbarTrack(anchor, xoffset, yoffset);
+
+  // Run to completion and check that we are around 3x pageScrollAmount, with
+  // some allowance for fractional rounding.
+  for (let i = 0; i < 1000; i++) {
+    utils.advanceTimeAndRefresh(16);
+  }
+  await promiseOnlyApzControllerFlushed();
+  curPos = scroller.scrollTop;
+  ok(Math.abs(curPos - (pageScrollAmount * 3)) < 3,
+     `Final scroll offset ${curPos} is close to 3x${pageScrollAmount}`);
+
+  utils.restoreNormalRefresh();
+}
+
+waitUntilApzStable()
+.then(test)
+.then(subtestDone, subtestFailed);
+
+  </script>
+</head>
+<body>
+ <div style="position:fixed; bottom: 0; right: 0; width: 1px; height: 1px" id="anchor"></div>
+ <div style="height: 300vh; margin-bottom: 10000px; background-image: linear-gradient(red,blue)"></div>
+  The above div is sized to 3x screen height so the linear gradient is more steep in terms of
+  color/pixel. We only scroll a few pages worth so we don't need the gradient all the way down.
+  And then we use a bottom-margin to make the page really big so the scrollthumb is
+  relatively small, giving us lots of space to click on the scrolltrack.
+</body>
+</html>