Let sticky elements be layout shift roots

This makes PaintInvalidator to detect shifts of descendants of a sticky ancestor based on the offset from the ancestor instead of the LayoutView. Bug: 1145078 Change-Id: I2887dd29e5ab7e7ad843d27673ad567775d4ecc1 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519360 Reviewed-by: Steve Kobes <skobes@chromium.org> Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org> Cr-Commit-Position: refs/heads/master@{#824473}
web-platform-tests · Nov 5, 2020 · d64a0cd · d64a0cd
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diff --git a/layout-instability/sticky-descendant-move.html b/layout-instability/sticky-descendant-move.html
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+<!DOCTYPE html>
+<title>Layout Instability: movement of descendant of sticky positioned</title>
+<link rel="help" href="https://wicg.github.io/layout-instability/" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/util.js"></script>
+<div style="position: sticky; width: 400px; height: 300px; top: 0">
+  <div id="child" style="position: relative; width: 300px; height: 200px; background: yellow"></div>
+</div>
+<script>
+promise_test(async () => {
+  const watcher = new ScoreWatcher;
+
+  // Wait for the initial render to complete.
+  await waitForAnimationFrames(2);
+
+  // Modify the position of the div.
+  const child = document.querySelector("#child");
+  child.style.top = '100px';
+
+  const expectedScore = computeExpectedScore(300 * (200 + 100), 100);
+
+  // Observer fires after the frame is painted.
+  assert_equals(watcher.score, 0);
+  await watcher.promise;
+  assert_equals(watcher.score, expectedScore);
+}, 'Movement of descendant of sticky positioned.');
+
+</script>
diff --git a/layout-instability/sticky-layout-no-change.html b/layout-instability/sticky-layout-no-change.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<title>Layout Instability: sticky positioned layout no change</title>
+<link rel="help" href="https://wicg.github.io/layout-instability/" />
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/util.js"></script>
+<div style="height: 3000px"></div>
+<div id="sticky" style="position: sticky; width: 200px; height: 300px; bottom: 0">
+  <div style="will-change: transform; height: 3000px; background: yellow"></div>
+</div>
+<script>
+
+promise_test(async () => {
+  const watcher = new ScoreWatcher;
+
+  // Wait for the initial render to complete.
+  await waitForAnimationFrames(2);
+
+  // This doesn't change layout because the sticky element sticks to the bottom.
+  sticky.style.marginTop = "-1000px";
+
+  await waitForAnimationFrames(3);
+  assert_equals(watcher.score, 0);
+}, 'Sticky layout no change.');
+
+</script>