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diff --git a/gfx/layers/apz/test/mochitest/helper_bug1637113_main_thread_hit_test.html b/gfx/layers/apz/test/mochitest/helper_bug1637113_main_thread_hit_test.html
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+++ b/gfx/layers/apz/test/mochitest/helper_bug1637113_main_thread_hit_test.html
@@ -0,0 +1,70 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1637113
+-->
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>Test for Bug 1637113</title>
+  <script src="/tests/SimpleTest/paint_listener.js"></script>
+  <script src="/tests/SimpleTest/EventUtils.js"></script>
+  <script type="application/javascript" src="apz_test_native_event_utils.js"></script>
+  <script type="application/javascript" src="apz_test_utils.js"></script>
+  <style>
+    iframe {
+      margin-top: 1000px;
+    }
+  </style>
+</head>
+<body>
+  <iframe id="subframe" srcdoc="<div id='target' style='width:100px;height:100px;'>" width="100px" height="100px"></iframe>
+  <script type="application/javascript">
+
+async function test() {
+  let utils = SpecialPowers.getDOMWindowUtils(window);
+
+  // Reproducing the bug requires three ingredients:
+  //  1. A large layout viewport offset.
+  //  2. A large visual viewport offset relative to the layout viewport.
+  //  3. An event that's dispatched in the iframe's document.
+  // We make the first two happen by doing a large visual scroll that will
+  // also drag the layout viewport with it part of the way.
+  let visualScrollPromise = new Promise(resolve => {
+    window.visualViewport.addEventListener("scroll", resolve, { once: true });
+  });
+  utils.scrollToVisual(0, 900, utils.UPDATE_TYPE_MAIN_THREAD,
+                       utils.SCROLL_MODE_INSTANT);
+  await visualScrollPromise;
+  await promiseApzFlushedRepaints();
+
+  let target = subframe.contentWindow.document.getElementById("target");
+  // To get an event that's dispatched in the iframe's document,
+  // synthesize a native tap. This will synthesize three events:
+  // a mouse-move, a mouse-down, and a mouse-up. The mouse-move
+  // and mouse-down are dispatched in the root content document.
+  // The mouse-down causes the iframe to "capture" the mouse, which
+  // leads the mouse-up to be dispatched in the iframe's document
+  // instead. We listen for the mouse-up.
+  let mouseUpEvent = null;
+  let mouseUpPromise = new Promise(resolve => {
+    target.addEventListener("mouseup", function(e) {
+      mouseUpEvent = e;
+      resolve();
+    });
+  });
+
+  await synthesizeNativeTap(target, 10, 10);
+  await mouseUpPromise;
+
+  is(mouseUpEvent.target, target, "mouseup event targeted the correct element");
+}
+
+SpecialPowers.getDOMWindowUtils(window).setResolutionAndScaleTo(2.0);
+waitUntilApzStable()
+.then(test)
+.then(subtestDone, subtestFailed);
+
+  </script>
+</body>
+</html>