[webkit-changes] [WebKit/WebKit] 2f33fa: A fixed header inside overflow scroll with a trans...

[Simon Fraser]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: 2f33fa54c40506def565e6af8f393614d02b2804
      https://github.com/WebKit/WebKit/commit/2f33fa54c40506def565e6af8f393614d02b2804
  Author: Simon Fraser <simon.fraser at apple.com>
  Date:   2023-01-21 (Sat, 21 Jan 2023)

  Changed paths:
    A LayoutTests/platform/ios-wk2/scrollingcoordinator/scrolling-tree/fixed-inside-stacking-overflow-inside-transformed-expected.txt
    A LayoutTests/scrollingcoordinator/scrolling-tree/fixed-inside-stacking-overflow-inside-transformed-expected.txt
    A LayoutTests/scrollingcoordinator/scrolling-tree/fixed-inside-stacking-overflow-inside-transformed.html
    M Source/WebCore/rendering/RenderLayerCompositor.cpp

  Log Message:
  -----------
  A fixed header inside overflow scroll with a transformed ancestor stutters on scrolling (affects Libby app)
https://bugs.webkit.org/show_bug.cgi?id=250652
rdar://104095908

Reviewed by Alan Baradlay.

The Libby app has content which uses a stacking-context overflow:scroll with a position:fixed
descendant, and a CSS transform on an ancestor of the scroller. In this situation, we treat the
position:fixed as if it has position:absolute, but still need to use the correct containing block
for it (which is the enclosing transformed box).

Two fixes are required here. First, `RenderLayerCompositor::computeCoordinatedPositioningForLayer()`
needs to check if the position:fixed layer actually has fixed behavior (i.e. no transformed
ancestor) before the early return.

Second, the `traverseAncestorLayers()` helper only handled position:absolute containing block logic;
we need to fix it to also compute the correct containing block for position:fixed (which allows it
to find transformed ancestors of fixed).

Fixing `traverseAncestorLayers()` to have the correct containingBlock behavior for fixed layers
revealed a surprising behavior, which is that the deprecated CSS `clip` property on a
position:absolute element clips position:fixed descendants, which is odd because it's different from
how overflow works (https://github.com/w3c/csswg-drafts/issues/8336). So
RenderLayerCompositor::computeAncestorClippingStack() needs some special case code to detect this
case. This is tested by imported/blink/fast/css/fixed-overlaps-absolute-in-clip.html.

* LayoutTests/platform/ios-wk2/scrollingcoordinator/scrolling-tree/fixed-inside-stacking-overflow-inside-transformed-expected.txt: Added.
* LayoutTests/scrollingcoordinator/scrolling-tree/fixed-inside-stacking-overflow-inside-transformed-expected.txt: Added.
* LayoutTests/scrollingcoordinator/scrolling-tree/fixed-inside-stacking-overflow-inside-transformed.html: Added.
* Source/WebCore/rendering/RenderLayerCompositor.cpp:
(WebCore::traverseAncestorLayers):
(WebCore::RenderLayerCompositor::computeAncestorClippingStack const):
(WebCore::RenderLayerCompositor::computeCoordinatedPositioningForLayer const):

Canonical link: https://commits.webkit.org/259175@main