[css-anchor-position-1] Scroll adjustment not applied for position:fixed anchored boxes #47291

anttijk · 2025-06-27T11:01:57Z

`de4580f`

[css-anchor-position-1] Scroll adjustment not applied for position:fixed anchored boxes
https://bugs.webkit.org/show_bug.cgi?id=295002
rdar://154348421

Reviewed by Simon Fraser.

Compute scroll adjustment correctly for fixed positioned anchored boxes.

* LayoutTests/TestExpectations:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-006-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-002-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-004-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-005-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-006-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-007-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-008-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-009-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-010-expected.txt:
* Source/WebCore/page/LocalFrameView.cpp:
(WebCore::LocalFrameView::addViewportConstrainedObject):
(WebCore::LocalFrameView::removeViewportConstrainedObject):
(WebCore::LocalFrameView::hasAnchorPositionedViewportConstrainedObjects const):

See if any of the viewport constrained objects are anchor positioned.

(WebCore::LocalFrameView::clearCachedHasAnchorPositionedViewportConstrainedObjects):
(WebCore::LocalFrameView::shouldUpdateCompositingLayersAfterScrolling const):

We need to update compositing layers on every frame if there are fixed anchor positioned boxes.

(WebCore::LocalFrameView::updateAnchorPositionedAfterScroll):

We need to update anchor positions after document scroll too.

* Source/WebCore/page/LocalFrameView.h:
* Source/WebCore/platform/ScrollableArea.cpp:
(WebCore::ScrollableArea::scrollPositionChanged):
* Source/WebCore/platform/ScrollableArea.h:
(WebCore::ScrollableArea::updateAnchorPositionedAfterScroll):

Factor into a separate virtual function, invoked from ScrollableArea::scrollPositionChanged.

* Source/WebCore/rendering/RenderBox.cpp:
(WebCore::RenderBox::styleWillChange):

Clear the cached fixed-with-anchor bit if anchor positioned style changes.

* Source/WebCore/rendering/RenderLayer.cpp:
(WebCore::RenderLayer::updateLayerPositionsAfterOverflowScroll):
(WebCore::RenderLayer::updateLayerPositionsAfterDocumentScroll):
* Source/WebCore/rendering/RenderLayerScrollableArea.cpp:
(WebCore::RenderLayerScrollableArea::scrollTo):

Move to RenderLayer::updateLayerPositionsAfterOverflowScroll.

* Source/WebCore/style/AnchorPositionEvaluator.cpp:
(WebCore::Style::scrollOffsetFromAnchor):

Take fixed position into account when computing scroll offset.

(WebCore::Style::AnchorPositionEvaluator::updateSnapshottedScrollOffsets):

Use defaultAnchorForBox() to figure out if we need scroll compensation.

(WebCore::Style::AnchorPositionEvaluator::updatePositionsAfterScroll):
(WebCore::Style::AnchorPositionEvaluator::isAnchorPositioned):
(WebCore::Style::scrollOffsetFromAncestorContainer): Deleted.
(WebCore::Style::AnchorPositionEvaluator::updateAfterOverflowScroll): Deleted.
* Source/WebCore/style/AnchorPositionEvaluator.h:

Canonical link: https://commits.webkit.org/296811@main

e52b786

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	✅ 🛠 playstation
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

webkit-early-warning-system · 2025-06-27T11:02:05Z

EWS run on previous version of this PR (hash b7c6dc0)

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	⏳ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	~~🧪 wpe-wk2~~	⏳ 🧪 win-tests
✅ 🧪 webkitperl	~~🧪 ios-wk2~~	~~🧪 api-mac~~	~~🧪 api-wpe~~
	⏳ 🧪 ios-wk2-wpt	~~🧪 mac-wk1~~	✅ 🛠 wpe-cairo
	~~🧪 api-ios~~	~~🧪 mac-wk2~~	✅ 🛠 gtk
	✅ 🛠 vision	⏳ 🧪 mac-AS-debug-wk2	~~🧪 gtk-wk2~~
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	~~🧪 api-gtk~~
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	⏳ 🛠 playstation
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

webkit-early-warning-system · 2025-06-27T11:23:22Z

EWS run on previous version of this PR (hash 87b333b)

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	❌ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	✅ 🛠 playstation
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

smfr · 2025-06-27T16:48:34Z

Source/WebCore/rendering/RenderElement.cpp

@@ -1006,6 +1006,9 @@ void RenderElement::styleWillChange(StyleDifference diff, const RenderStyle& new
        if (drawsRootBackground && newStyle.hasEntirelyFixedBackground() && view().compositor().supportsFixedRootBackgroundCompositing())
            newStyleSlowScroll = false;
    }
+    // FIXME: Scrolling tree support for anchor positioning.
+    if (newStyle.position() == PositionType::Fixed && Style::AnchorPositionEvaluator::isAnchorPositioned(newStyle))


What about sticky positioning?

Sticky can't be anchor positioned, only fixed and absolute.

smfr · 2025-06-27T16:49:48Z

Source/WebCore/rendering/RenderElement.cpp

@@ -1176,7 +1179,7 @@ void RenderElement::willBeDestroyed()
    if (!renderTreeBeingDestroyed() && element())
        document().contentChangeObserver().rendererWillBeDestroyed(*element());
 #endif
-    if (m_style.hasAnyFixedBackground() && !settings().fixedBackgroundsPaintRelativeToDocument())
+    if ((m_style.hasAnyFixedBackground() && !settings().fixedBackgroundsPaintRelativeToDocument()) || (m_style.position() == PositionType::Fixed && Style::AnchorPositionEvaluator::isAnchorPositioned(m_style)))
        view().protectedFrameView()->removeSlowRepaintObject(*this);


It's not ideal to use the "slow repaint" logic for this; this is about needing to layout after scroll, not about painting. This is more like the setViewportConstrainedObjectsNeedLayout code path.

This is about needing compositing update for every frame.

Basically LocalFrameView::shouldUpdateCompositingLayersAfterScrolling() needs to return true in this case.

Did a version using viewport constrained objects.

smfr · 2025-06-27T16:51:17Z

Source/WebCore/rendering/RenderLayerCompositor.cpp

@@ -6012,6 +6013,9 @@ void RenderLayerCompositor::updateSynchronousScrollingNodes()
        } else if (!layer->behavesAsFixed()) {
            LOG_WITH_STREAM(Scrolling, stream << "RenderLayerCompositor::updateSynchronousScrollingNodes - root node slow-scrolling because of fixed backgrounds");
            rootHasSlowRepaintObjects = true;
+        } else if (Style::AnchorPositionEvaluator::isAnchorPositioned(renderer.style())) {
+            LOG_WITH_STREAM(Scrolling, stream << "RenderLayerCompositor::updateSynchronousScrollingNodes - root node slow-scrolling because of a fixed anchored element");
+            rootHasSlowRepaintObjects = true;


Again, mixing layout with repainting.

smfr · 2025-06-27T16:51:36Z

Source/WebCore/style/AnchorPositionEvaluator.cpp


    auto offset = LayoutSize { };
-    for (auto* ancestor = descendant.container(); ancestor && ancestor != &ancestorContainer; ancestor = ancestor->container()) {
+    bool isFixedAnchor = anchor.isFixedPositioned();


Need to consider sticky.

smfr · 2025-06-27T16:51:51Z

Source/WebCore/style/AnchorPositionEvaluator.cpp

-    for (auto* ancestor = descendant.container(); ancestor && ancestor != &ancestorContainer; ancestor = ancestor->container()) {
+    bool isFixedAnchor = anchor.isFixedPositioned();
+
+    for (auto* ancestor = anchor.container(); ancestor && ancestor != containingBlock; ancestor = ancestor->container()) {


CheckedPtr?

webkit-early-warning-system · 2025-06-30T12:48:48Z

EWS run on previous version of this PR (hash 6e95cfc)

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	~~🧪 wpe-wk2~~	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	~~🧪 api-mac~~	~~🧪 api-wpe~~
	✅ 🧪 ios-wk2-wpt	~~🧪 mac-wk1~~	✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	~~🧪 mac-wk2~~	✅ 🛠 gtk
	✅ 🛠 vision	~~🧪 mac-AS-debug-wk2~~	~~🧪 gtk-wk2~~
	✅ 🛠 vision-sim	~~🧪 mac-wk2-stress~~	~~🧪 api-gtk~~
	~~🧪 vision-wk2~~	~~🧪 mac-intel-wk2~~	✅ 🛠 playstation
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

webkit-early-warning-system · 2025-06-30T13:07:19Z

EWS run on current version of this PR (hash e52b786)

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2	⏳ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-cairo
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2	✅ 🛠 playstation
	✅ 🛠 tv	✅ 🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

…xed anchored boxes https://bugs.webkit.org/show_bug.cgi?id=295002 rdar://154348421 Reviewed by Simon Fraser. Compute scroll adjustment correctly for fixed positioned anchored boxes. * LayoutTests/TestExpectations: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-006-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-002-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-004-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-005-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-006-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-007-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-008-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-009-expected.txt: * LayoutTests/imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-position-try-010-expected.txt: * Source/WebCore/page/LocalFrameView.cpp: (WebCore::LocalFrameView::addViewportConstrainedObject): (WebCore::LocalFrameView::removeViewportConstrainedObject): (WebCore::LocalFrameView::hasAnchorPositionedViewportConstrainedObjects const): See if any of the viewport constrained objects are anchor positioned. (WebCore::LocalFrameView::clearCachedHasAnchorPositionedViewportConstrainedObjects): (WebCore::LocalFrameView::shouldUpdateCompositingLayersAfterScrolling const): We need to update compositing layers on every frame if there are fixed anchor positioned boxes. (WebCore::LocalFrameView::updateAnchorPositionedAfterScroll): We need to update anchor positions after document scroll too. * Source/WebCore/page/LocalFrameView.h: * Source/WebCore/platform/ScrollableArea.cpp: (WebCore::ScrollableArea::scrollPositionChanged): * Source/WebCore/platform/ScrollableArea.h: (WebCore::ScrollableArea::updateAnchorPositionedAfterScroll): Factor into a separate virtual function, invoked from ScrollableArea::scrollPositionChanged. * Source/WebCore/rendering/RenderBox.cpp: (WebCore::RenderBox::styleWillChange): Clear the cached fixed-with-anchor bit if anchor positioned style changes. * Source/WebCore/rendering/RenderLayer.cpp: (WebCore::RenderLayer::updateLayerPositionsAfterOverflowScroll): (WebCore::RenderLayer::updateLayerPositionsAfterDocumentScroll): * Source/WebCore/rendering/RenderLayerScrollableArea.cpp: (WebCore::RenderLayerScrollableArea::scrollTo): Move to RenderLayer::updateLayerPositionsAfterOverflowScroll. * Source/WebCore/style/AnchorPositionEvaluator.cpp: (WebCore::Style::scrollOffsetFromAnchor): Take fixed position into account when computing scroll offset. (WebCore::Style::AnchorPositionEvaluator::updateSnapshottedScrollOffsets): Use defaultAnchorForBox() to figure out if we need scroll compensation. (WebCore::Style::AnchorPositionEvaluator::updatePositionsAfterScroll): (WebCore::Style::AnchorPositionEvaluator::isAnchorPositioned): (WebCore::Style::scrollOffsetFromAncestorContainer): Deleted. (WebCore::Style::AnchorPositionEvaluator::updateAfterOverflowScroll): Deleted. * Source/WebCore/style/AnchorPositionEvaluator.h: Canonical link: https://commits.webkit.org/296811@main

webkit-commit-queue · 2025-06-30T17:57:11Z

Committed 296811@main (de4580f): https://commits.webkit.org/296811@main

Reviewed commits have been landed. Closing PR #47291 and removing active labels.

anttijk self-assigned this Jun 27, 2025

anttijk added the Layout and Rendering For bugs with layout and rendering of Web pages. label Jun 27, 2025

anttijk force-pushed the anchor-scroll-fixed branch from b7c6dc0 to 87b333b Compare June 27, 2025 11:23

smfr reviewed Jun 27, 2025

View reviewed changes

anttijk force-pushed the anchor-scroll-fixed branch from 87b333b to 6e95cfc Compare June 30, 2025 12:48

anttijk requested a review from cdumez as a code owner June 30, 2025 12:48

anttijk force-pushed the anchor-scroll-fixed branch from 6e95cfc to e52b786 Compare June 30, 2025 13:07

smfr approved these changes Jun 30, 2025

View reviewed changes

anttijk added the merge-queue Applied to send a pull request to merge-queue label Jun 30, 2025

webkit-commit-queue force-pushed the anchor-scroll-fixed branch from e52b786 to de4580f Compare June 30, 2025 17:57

webkit-commit-queue merged commit de4580f into WebKit:main Jun 30, 2025

webkit-commit-queue removed the merge-queue Applied to send a pull request to merge-queue label Jun 30, 2025

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[css-anchor-position-1] Scroll adjustment not applied for position:fixed anchored boxes #47291

[css-anchor-position-1] Scroll adjustment not applied for position:fixed anchored boxes #47291

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[css-anchor-position-1] Scroll adjustment not applied for position:fixed anchored boxes #47291

[css-anchor-position-1] Scroll adjustment not applied for position:fixed anchored boxes #47291

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