<!DOCTYPE html>
<title>Nested top-constrainted position:sticky elements should render correctly</title>
<link rel="help" href="https://www.w3.org/TR/css-position-3/#sticky-pos" />
<meta name="assert" content="This test checks that nested position:sticky elements with a top constraint render correctly" />

<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 = setupNestedStickyTest('top', 50, 60);
  elements.scroller.scrollTop = 100;
  const nonStickyTopY = elements.container.offsetTop +
      elements.filler.clientHeight;
  assert_equals(elements.sticky.offsetTop, nonStickyTopY);
  // The inner sticky should not be offset from the outer.
  assert_equals(elements.innerSticky.offsetTop, 0);
}, 'before reaching the sticking point, neither sticky box should be offset');

test(() => {
  const elements = setupNestedStickyTest('top', 50, 60);
  elements.scroller.scrollTop = 145;
  const nonStickyTopY = elements.container.offsetTop +
      elements.filler.clientHeight;
  assert_equals(elements.sticky.offsetTop, nonStickyTopY);
  assert_equals(elements.innerSticky.offsetTop, 5);
}, 'the inner sticky can stick before the outer one if necessary');

test(() => {
  const elements = setupNestedStickyTest('top', 50, 60);
  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);

  // The inner sticky has similar math, but its offsetTop is relative to the
  // sticky element and in this test is the difference between the top values.
  assert_equals(elements.innerSticky.offsetTop, 10);
}, 'both sticky boxes can be stuck at the same time');

test(() => {
  const elements = setupNestedStickyTest('top', 50, 60);
  elements.scroller.scrollTop = 300;
  const maxOffsetInContainer = elements.container.offsetTop +
    elements.container.clientHeight - elements.sticky.clientHeight;
  assert_equals(elements.sticky.offsetTop, maxOffsetInContainer);
  const maxOffsetInOuterSticky = elements.sticky.clientHeight -
      elements.innerSticky.clientHeight;
  assert_equals(elements.innerSticky.offsetTop, maxOffsetInOuterSticky);
}, 'neither sticky can escape their containing block');

test(() => {
  const elements = setupNestedStickyTest('top', 50, 300);
  elements.scroller.scrollTop = 100;
  // The outer sticky has not stuck yet.
  const nonStickyTopY = elements.container.offsetTop +
      elements.filler.clientHeight;
  assert_equals(elements.sticky.offsetTop, nonStickyTopY);
  // But the inner sticky still cannot escape the outer sticky (as it is the
  // containing block).
  const maxOffsetInOuterSticky = elements.sticky.clientHeight -
      elements.innerSticky.clientHeight;
  assert_equals(elements.innerSticky.offsetTop, maxOffsetInOuterSticky);
}, 'the inner sticky cannot be pushed outside the outer sticky');
</script>