Bug 309429 – [Scroll anchoring] css/css-scroll-anchoring/position-change-heuristic.html is flaky

• Status: RESOLVED
• Resolution: FIXED
• Component: Scrolling

[Simon Fraser (smfr)]

css/css-scroll-anchoring/position-change-heuristic.html often fails with:

FAIL Position changes in document scroller. assert_equals: expected 225 but got 200

when running:

runCase from fixed  to absolute - expectSuppression false

[Radar WebKit Bug Importer]

<rdar://problem/171998007>

[Simon Fraser (smfr)]

This happens because the scroll anchoring logic runs in the middle of `updateScrollbars` in response to a content size change:

```
    frame #0: 0x000000030757b750 WebCore`WebCore::ScrollView::setScrollOffset(this=0x000000013e2c0800, offset={ x = 0, y = 225 }) at ScrollView.cpp:457:30
    frame #1: 0x00000003001ad9f8 WebCore`WebCore::ScrollableArea::scrollPositionChanged(this=0x000000013e2c0800, position={ x = 0, y = 225 }) at ScrollableArea.cpp:233:5
    frame #2: 0x0000000307581804 WebCore`WebCore::ScrollableArea::notifyScrollPositionChanged(this=0x000000013e2c0800, position={ x = 0, y = 225 }) at ScrollableArea.cpp:225:5
    frame #3: 0x00000003073ff0fc WebCore`WebCore::AsyncScrollingCoordinator::reconcileScrollingState(this=0x0000000118208220, frameView=0x000000013e2c0800, scrollPosition={ x = 0.0, y = 225.0 }, layoutViewportOriginOrOverrideRect=0x000000016ae8b8c0, scrollType=Programmatic, viewportRectStability=Stable, scrollingLayerPositionAction=Set) at AsyncScrollingCoordinator.cpp:879:15
    frame #4: 0x00000003073fe46c WebCore`WebCore::AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScroll(this=0x0000000118208220, nodeID=WebCore::ScrollingNodeID @ 0x000000016ae8b988, scrollPosition={ x = 0.0, y = 225.0 }, layoutViewportOrigin= Has Value=false , updateLayerPositionAction=Set, scrollType=Programmatic, viewportRectStability=Stable) at AsyncScrollingCoordinator.cpp:842:9
    frame #5: 0x00000003073fd924 WebCore`WebCore::AsyncScrollingCoordinator::applyScrollPositionUpdate(this=0x0000000118208220, update=0x000000016ae8bc90, scrollType=Programmatic, viewportStability=Stable) at AsyncScrollingCoordinator.cpp:707:13
    frame #6: 0x00000003073fe0f4 WebCore`WebCore::AsyncScrollingCoordinator::applyScrollUpdate(this=0x0000000118208220, update=0x000000016ae8bc90, scrollType=Programmatic, viewportStability=Stable) at AsyncScrollingCoordinator.cpp:696:5
    frame #7: 0x00000003073fbc88 WebCore`WebCore::AsyncScrollingCoordinator::requestScrollToPosition(this=0x0000000118208220, scrollableArea=0x000000013e2c0800, scrollPosition={ x = 0, y = 225 }, options=0x000000016ae8bfec) at AsyncScrollingCoordinator.cpp:413:9
    frame #8: 0x0000000307172528 WebCore`WebCore::LocalFrameView::requestScrollToPosition(this=0x000000013e2c0800, position={ x = 0, y = 225 }, options=0x000000016ae8bfec) at LocalFrameView.cpp:3894:38
    frame #9: 0x00000003074077b0 WebCore`WebCore::ScrollAnchoringController::adjustScrollPositionForAnchoring(this=0x00000001181cc180) at ScrollAnchoringController.cpp:659:34
    frame #10: 0x0000000307581ca0 WebCore`WebCore::ScrollableArea::adjustScrollAnchoringPosition(this=0x000000013e2c0800) at ScrollableArea.cpp:305:21
    frame #11: 0x0000000307176550 WebCore`WebCore::LocalFrameView::adjustScrollAnchoringPositionForScrollableAreas(this=0x000000013e2c0800) at LocalFrameView.cpp:4788:24
    frame #12: 0x0000000307175b4c WebCore`WebCore::LocalFrameView::performPostLayoutTasks(this=0x000000013e2c0800) at LocalFrameView.cpp:4725:5
    frame #13: 0x0000000307182a48 WebCore`WebCore::LocalFrameViewLayoutContext::runPostLayoutTasks(this=0x000000013e2c0970) at LocalFrameViewLayoutContext.cpp:332:22
    frame #14: 0x0000000307184698 WebCore`WebCore::LocalFrameViewLayoutContext::runOrScheduleAsynchronousTasks(this=0x000000013e2c0970, canDeferUpdateLayerPositions=false) at LocalFrameViewLayoutContext.cpp:318:5
    frame #15: 0x0000000307183bb8 WebCore`WebCore::LocalFrameViewLayoutContext::performLayout(this=0x000000013e2c0970, canDeferUpdateLayerPositions=false) at LocalFrameViewLayoutContext.cpp:294:9
    frame #16: 0x0000000307155fb4 WebCore`WebCore::LocalFrameViewLayoutContext::layout(this=0x000000013e2c0970, canDeferUpdateLayerPositions=false) at LocalFrameViewLayoutContext.cpp:158:5
    frame #17: 0x0000000307172c7c WebCore`WebCore::LocalFrameView::updateContentsSize(this=0x000000013e2c0800) at LocalFrameView.cpp:3995:25
  * frame #18: 0x00000003000ef9a4 WebCore`WebCore::ScrollView::updateScrollbars(this=0x000000013e2c0800, desiredPosition={ x = 0, y = 200 }) at ScrollView.cpp:733:13
    frame #19: 0x00000003000704ec WebCore`WebCore::ScrollView::setContentsSize(this=0x000000013e2c0800, newSize={ width = 793, height = 4016 }) at ScrollView.cpp:412:9
    frame #20: 0x000000030715ce68 WebCore`WebCore::LocalFrameView::setContentsSize(this=0x000000013e2c0800, size={ width = 793, height = 4016 }) at LocalFrameView.cpp:550:17
    frame #21: 0x0000000307152acc WebCore`WebCore::LocalFrameView::adjustViewSize(this=0x000000013e2c0800) at LocalFrameView.cpp:586:5
    frame #22: 0x0000000307183a34 WebCore`WebCore::LocalFrameViewLayoutContext::performLayout(this=0x000000013e2c0970, canDeferUpdateLayerPositions=false) at LocalFrameViewLayoutContext.cpp:281:30
    frame #23: 0x0000000307155fb4 WebCore`WebCore::LocalFrameViewLayoutContext::layout(this=0x000000013e2c0970, canDeferUpdateLayerPositions=false) at LocalFrameViewLayoutContext.cpp:158:5
    frame #24: 0x0000000305d83138 WebCore`WebCore::Document::updateLayout(this={ origin = , url = , inMainFrame = Detached, backForwardCacheState = NotInBackForwardCache }, layoutOptions={ size = 3 }, context=0x00000001182f80e0) at Document.cpp:3168:32
    frame #25: 0x0000000305d7f05c WebCore`WebCore::Document::updateLayoutIgnorePendingStylesheets(this={ origin = , url = , inMainFrame = Detached, backForwardCacheState = NotInBackForwardCache }, layoutOptions={ size = 3 }, context=0x00000001182f80e0) at Document.cpp:3049:12
    frame #26: 0x0000000305ef6d70 WebCore`WebCore::Element::scrollTop(this=0x00000001182f80e0) at Element.cpp:1707:15
    frame #27: 0x0000000301896210 WebCore`WebCore::jsElement_scrollTopGetter(lexicalGlobalObject=0x000000013ec20088, thisObject=0x000000011838ddc8) at JSElement.cpp:3231:89
```

but ScrollView::updateScrollbars stores the current scroll position in `desiredPosition` so clobbers the scroll position computed for anchoring.

[Simon Fraser (smfr)]

Pull request: https://github.com/WebKit/WebKit/pull/60144

[EWS]

Committed 308920@main (96c4a7386d59): <https://commits.webkit.org/308920@main>

Reviewed commits have been landed. Closing PR #60144 and removing active labels.