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Bug 1905426 - Refine zero-scroll-offset scroll anchoring heuristic to match blink. r=hiro, a=dmeehan

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See link in the comment. We were checking both offsets, but blink only
checks one.

In this case, the page has a table with `overflow-x: auto` and
`overflow-y: hidden`. They implement a custom virtual scrolling on the
vertical axis, and scroll-anchoring interacts poorly with it.

Adapt zero-scroll-offset.html to test for this.

Differential Revision: https://phabricator.services.mozilla.com/D217201
This commit is contained in:
Emilio Cobos Álvarez 2024-07-22 22:45:49 +00:00
parent 32ec780a99
commit 1c40fa8a53
3 changed files with 64 additions and 3 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

11
layout/generic/ScrollAnchorContainer.cpp
View file

@ -230,11 +230,16 @@ bool ScrollAnchorContainer::CanMaintainAnchor() const {
return false;
}
// Or if the scroll frame has not been scrolled from the logical origin. This
// is not in the specification [1], but Blink does this.
// Or if the scroll frame has not been scrolled from the logical origin of the
// block axis. This is not in the specification [1], but Blink does this [2].
//
// [1] https://github.com/w3c/csswg-drafts/issues/3319
if (Frame()->GetLogicalScrollPosition() == nsPoint()) {
// [2]
// https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/scroll_anchor.cc;l=551;drc=f1eab630d343484302ee9bea91f515f1a1dd0891
const nsPoint pos = Frame()->GetLogicalScrollPosition();
const nscoord blockOffset =
Frame()->GetWritingMode().IsVertical() ? pos.x : pos.y;
if (blockOffset == 0) {
return false;
}

0
testing/web-platform/tests/css/css-scroll-anchoring/zero-scroll-offset.html → testing/web-platform/tests/css/css-scroll-anchoring/zero-scroll-offset-001.html
View file

56
testing/web-platform/tests/css/css-scroll-anchoring/zero-scroll-offset-002.html Normal file
View file

@ -0,0 +1,56 @@
<!DOCTYPE html>
<head>
<title>Test that scroll anchoring is suppressed when scroll offset is zero in the block axis, even if the inline axis scroll offset isn't.</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@mozilla.com">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring/">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1905426">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<style>
#header {
height: 100px;
border: 1px solid black;
overflow-anchor: none;
}
#content {
height: 200vh;
width: 200vw;
}
</style>
<div id="header"></div>
<div id="content">abc</div>
<script>
window.addEventListener("scroll", function() {
if (document.scrollingElement.scrollTop > 0) {
// On the first scroll event, shrink the header. Scroll anchoring anchors to
// content, but the header shrinks by more than the scroll offset so the
// resulting scroll position is zero.
step_timeout(function() {
document.querySelector("#header").style.height = "50px";
}, 0);
} else {
// On the second scroll event, grow the header. Since the scroll offset is
// zero, scroll anchoring should be suppressed. Otherwise, scroll anchoring
// would anchor to content and the resulting scroll position would be 50px.
step_timeout(function() {
document.querySelector("#header").style.height = "100px";
}, 0);
}
});
async_test(function(t) {
// Scroll down a bit to trigger the scroll event listener.
window.scrollTo(10, 10);
window.requestAnimationFrame(function() {
window.requestAnimationFrame(function() {
window.requestAnimationFrame(t.step_func_done(() => {
assert_equals(document.scrollingElement.scrollTop, 0);
}));
});
});
}, "Scroll anchoring suppressed when scroll offset is zero.");
</script>