visual-viewport/viewport-read-size-in-iframe-causes-layout.html

<!DOCTYPE html>
<meta name="viewport" content="width=device-width, minimum-scale=1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  iframe {
    width: 200px;
    height: 300px;
  }
</style>

<h4>This test checks that requesting the viewport size in an iframe causes any pending layout to occur.</h4>
<iframe srcdoc="<!DOCTYPE html><style>html{height:100%}</style>"></iframe>
<script>
  async_test(function(t) {
    window.onload = t.step_func(function() {
      assert_equals(frames[0].window.visualViewport.width, 200,
          "Reading width of iframe viewport should match iframe width.");
      assert_equals(frames[0].window.visualViewport.height, 300,
          "Reading height of iframe viewport should match iframe height.");

      // Add overflow so scrollbars appear.
      window.frames[0].window.document.body.style.width = "2000px";
      window.frames[0].window.document.body.style.height = "2000px";

      var viewportWidth = frames[0].window.visualViewport.width;
      var viewportHeight = frames[0].window.visualViewport.height;

      assert_equals(viewportWidth, frames[0].window.document.documentElement.clientWidth,
          "Reading width of iframe viewport should cause a layout and exclude the new scrollbar.");
      assert_equals(viewportHeight, frames[0].window.document.documentElement.clientHeight,
          "Reading height of iframe viewport should cause a layout and exclude the new scrollbar.");
      t.done();
    });
  });
</script>