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chromium/third_party/blink/web_tests/external/wpt/css/css-position/sticky/position-sticky-top.html 

<!DOCTYPE html>
<title>position:sticky elements should respect the top constraint</title>
<link rel="help" href="https://www.w3.org/TR/css-position-3/#sticky-pos" />
<meta name="assert" content="This test checks that position:sticky elements obey their top anchor after scrolling" />

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

<script src="../resources/sticky-util.js"></script>

<body></body>

<script>
test(() => {
  const elements = setupStickyTest('top', 50);
  elements.scroller.scrollTop = 100;
  const nonStickyTopY = elements.container.offsetTop +
      elements.filler.clientHeight;
  assert_equals(elements.sticky.offsetTop, nonStickyTopY);
}, 'before reaching the sticking point the sticky box should not be offset');

test(() => {
  const elements = setupStickyTest('top', 50);
  elements.scroller.scrollTop = 200;

  // This math cancels to sticky.offsetTop == (scroller.scrollTop + 50), but
  // for clarity the calculations are left explicit.
  const nonStickyTopY = elements.container.offsetTop +
      elements.filler.clientHeight;
  const targetTopY = elements.scroller.scrollTop + 50;
  const stickyOffset = targetTopY - nonStickyTopY;

  assert_equals(elements.sticky.offsetTop, nonStickyTopY + stickyOffset);
}, 'after reaching the sticking point the sticky box should be offset');

test(() => {
  const elements = setupStickyTest('top', 50);
  elements.scroller.scrollTop = 300;
  const maxOffsetInContainer = elements.container.offsetTop +
      elements.container.clientHeight - elements.sticky.clientHeight;
  assert_equals(elements.sticky.offsetTop, maxOffsetInContainer);
}, 'the sticky box should not be pushed outside its containing block');
</script>