Branch: refs/heads/main
Home: https://github.com/WebKit/WebKit
Commit: 2f33fa54c40506def565e6af8f393614d02b2804
https://github.com/WebKit/WebKit/commit/2f33fa54c40506def565e6af8f393614d02b2804
Author: Simon Fraser <[email protected]>
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
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