chromium/src/third_party/blink/web_tests/fast/events/scroll-in-scaled-page-with-overflow-hidden.html

<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../../resources/gesture-util.js"></script>

<style>
    html, body { margin:0; overflow: hidden; }
</style>

<div>
  This tests that a usually overflow: hidden viewport should be scrollable when
  scaled. Otherwise, you can't get to content you would have been able to get to
  had you not been zoomed it.
</div>
<div id="console"></div>

<div style='font-size:40px; height: 1000px;'>
</div>

<script>
  const MOUSE_INPUT = GestureSourceType.MOUSE_INPUT;
  async function runTest() {
    promise_test( async () => {
      assert_true(typeof(window.internals) !== "undefined",
                  "This test requires window.internals.");

      // Force a layout.
      document.body.offsetLeft;

      internals.setPageScaleFactor(2);

      await waitForCompositorCommit();

      // Note that with ScrollTopLeftInterop enabled,
      // document.scrollingElement is null because document.body is
      // potentially scrollable (with overflow:hidden the box can still
      // be scrolled programmatically).
      assert_equals(window.visualViewport.pageTop, 0);

      await smoothScroll(700, 100, 100, MOUSE_INPUT, 'down', 3000);
      await waitFor(() => { return window.visualViewport.pageTop >= 300; });
      assert_equals(window.visualViewport.pageTop,
                    300,
                    "Visual viewport is scrolled fully to the bottom");
    },
    "Viewport with overflow: hidden can still be panned when pinch-zoomed in.");
  }
  window.addEventListener("load", runTest);
</script>