[webkit-changes] [WebKit/WebKit] 416e20: REGRESSION (265576 at main): Wrong paint order of pos...

[Simon Fraser]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: 416e2026cb6c5117a75c93627cc0634a2634a38f
      https://github.com/WebKit/WebKit/commit/416e2026cb6c5117a75c93627cc0634a2634a38f
  Author: Simon Fraser <simon.fraser at apple.com>
  Date:   2023-09-15 (Fri, 15 Sep 2023)

  Changed paths:
    M LayoutTests/compositing/layer-creation/clipping-scope/nested-scroller-overlap-expected.txt
    A LayoutTests/compositing/shared-backing/overflow-scroll/absolute-in-nested-scroller-paint-order-expected.html
    A LayoutTests/compositing/shared-backing/overflow-scroll/absolute-in-nested-scroller-paint-order.html
    A LayoutTests/compositing/shared-backing/overflow-scroll/nested-scroller-sharing-expected.txt
    A LayoutTests/compositing/shared-backing/overflow-scroll/nested-scroller-sharing.html
    A LayoutTests/compositing/shared-backing/overflow-scroll/overlapping-scroller-sharing-expected.txt
    A LayoutTests/compositing/shared-backing/overflow-scroll/overlapping-scroller-sharing.html
    M LayoutTests/platform/ios-wk2/compositing/layer-creation/clipping-scope/nested-scroller-overlap-expected.txt
    A LayoutTests/platform/ios-wk2/compositing/shared-backing/overflow-scroll/nested-scroller-sharing-expected.txt
    A LayoutTests/platform/ios-wk2/compositing/shared-backing/overflow-scroll/overlapping-scroller-sharing-expected.txt
    M Source/WebCore/rendering/RenderLayerCompositor.cpp

  Log Message:
  -----------
  REGRESSION (265576 at main): Wrong paint order of positioned and in-flow content inside overflow:scroll
https://bugs.webkit.org/show_bug.cgi?id=261302
rdar://115144982

Reviewed by Antti Koivisto.

Backing sharing is used to reduce memory use by allowing multiple RenderLayers (normally siblings)
which share the same stacking context ancestor to render into the same compositing layer. This has
to be done in a way that preserves back-to-front paint order. The common case where this kicks in is
a non-stacking context overflow:scroll with position:relative descendants.

265576 at main added support for multiple backing sharing providers, but in a way that broke certain
configurations, particularly with nested scrollers; the specific bug here was that we computed that
the abspos "header" element could share backing with the parent scroller, but that had it painting
behind the child scroller.

Getting backing sharing correct with overlapping and nested scrollers is hard, so this patch limits
multiple backing sharing providers to non-overlapping overflow scrollers only (which still fixes
walmart.com, the target of the original fix). A long comment was added to explain the complexities
of these configurations.

Additional cleanup fixes: "continueBackingSharingSequence" is renamed to
"addBackingSharingCandidate", and "canUseMultipleProviders" to "isAdditionalProviderCandidate". The
fix is to return false from "isAdditionalProviderCandidate" when things overlap.

* LayoutTests/compositing/layer-creation/clipping-scope/nested-scroller-overlap-expected.txt:
* LayoutTests/compositing/shared-backing/overflow-scroll/absolute-in-nested-scroller-paint-order-expected.html: Added.
* LayoutTests/compositing/shared-backing/overflow-scroll/absolute-in-nested-scroller-paint-order.html: Added.
* LayoutTests/compositing/shared-backing/overflow-scroll/nested-scroller-sharing-expected.txt: Copied from LayoutTests/platform/ios-wk2/compositing/layer-creation/clipping-scope/nested-scroller-overlap-expected.txt.
* LayoutTests/compositing/shared-backing/overflow-scroll/nested-scroller-sharing.html: Added.
* LayoutTests/compositing/shared-backing/overflow-scroll/overlapping-scroller-sharing-expected.txt: Added.
* LayoutTests/compositing/shared-backing/overflow-scroll/overlapping-scroller-sharing.html: Added.
* LayoutTests/platform/ios-wk2/compositing/layer-creation/clipping-scope/nested-scroller-overlap-expected.txt:
* LayoutTests/platform/ios-wk2/compositing/shared-backing/overflow-scroll/nested-scroller-sharing-expected.txt: Added.
* LayoutTests/platform/ios-wk2/compositing/shared-backing/overflow-scroll/overlapping-scroller-sharing-expected.txt: Added.
* Source/WebCore/rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::BackingSharingState::firstProviderCandidateLayer const):
(WebCore::RenderLayerCompositor::BackingSharingState::startBackingSharingSequence):
(WebCore::RenderLayerCompositor::BackingSharingState::addBackingSharingCandidate):
(WebCore::RenderLayerCompositor::BackingSharingState::endBackingSharingSequence):
(WebCore::RenderLayerCompositor::BackingSharingState::backingProviderCandidateForLayer):
(WebCore::RenderLayerCompositor::BackingSharingState::isAdditionalProviderCandidate const):
(WebCore::RenderLayerCompositor::BackingSharingState::updateBeforeDescendantTraversal):
(WebCore::RenderLayerCompositor::BackingSharingState::updateAfterDescendantTraversal):
(WebCore::RenderLayerCompositor::computeCompositingRequirements):
(WebCore::RenderLayerCompositor::BackingSharingState::continueBackingSharingSequence): Deleted.
(WebCore::RenderLayerCompositor::BackingSharingState::canUseMultipleProviders const): Deleted.

Canonical link: https://commits.webkit.org/268028@main