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chromium/third_party/blink/web_tests/external/wpt/selection/textcontrols/focus.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Stop selection extension when focus changes</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-actions.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<link rel="stylesheet" href="/fonts/ahem.css">
<style>
#p {
  font: 16px/1 Ahem;
}
</style>
<p id="p">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna
  aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
  Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint
  occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
</p>
<textarea id="textarea">Hello</textarea>
<script>
  const selection = getSelection();
  const { p, textarea } = document.all;
  document.addEventListener("selectionchange", ev => {
    if (p.contains(selection.focusNode)) {
      textarea.focus();
    }
  });

  promise_test(async () => {
    await new test_driver.Actions()
      .pointerMove(5, 5, {origin: p})
      .pointerDown()
      .pointerMove(50, 50)
      .pointerUp()
      .send();
    assert_equals(selection.focusNode, document.body);
    assert_equals(selection.focusOffset, 2);
    assert_equals(selection.anchorNode, document.body);
    assert_equals(selection.focusOffset, 2);
  }, "focus() should cancel selection extension by pointer device");
</script>