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+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width; initial-scale=1.0">
+  <title>Test for bug 1827330</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>
+  <style>
+    #scrollbox {
+      overflow: scroll;
+      width: 300px;
+      height: 300px;
+    }
+    .spacer {
+      height: 200%;
+    }  </style>
+</head>
+<body>
+  <div id="scrollbox">
+    <iframe src="https://example.com" width="200" height="200"></iframe>
+    <div class="spacer"></div>
+  </div>
+  <script type="text/javascript">
+
+    async function test() {
+      const utils = SpecialPowers.getDOMWindowUtils(window);
+
+      // Zoom in.
+      let resolution = 3.0;
+      utils.setResolutionAndScaleTo(resolution);
+      await promiseApzFlushedRepaints();
+
+      var subframe = document.getElementById("scrollbox");
+      let scrollPromise = new Promise(resolve => {
+        subframe.addEventListener("scroll", resolve, {once: true});
+      });
+
+      // Scroll down a small amount (5px). The bug in this case is that the
+      // scroll thumb does not start moving until the mouse has already moved
+      // by some distance, so if the bug occurs, the first 5px of mouse movement
+      // will cause no scrolling.
+      var dragFinisher = await promiseVerticalScrollbarDrag(subframe, 5, 5);
+      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();
+
+      // Check that we've scrolled at all.
+      ok(subframe.scrollTop > 0, "Scrollbar drag resulted in a scroll position of " + subframe.scrollTop);
+    }
+
+    waitUntilApzStable()
+    .then(test)
+    .then(subtestDone, subtestFailed);
+
+  </script>
+</body>
+</html>