Sometimes the contents of a visibility:hidden iframe become visible

[smfr]

d927d70

Sometimes the contents of a visibility:hidden iframe become visible
https://bugs.webkit.org/show_bug.cgi?id=272727
rdar://126178201

Reviewed by Matt Woodrow.

A normal composited but `visibility:hidden` layer uses `setContentsVisible(false)`
to hide its contents (by clearing the layer's `contents` property), and this works
within a document because `visibility` is inherited, and allows a visible child
inside a hidden ancestor.

For plugin or iframe layers we try to avoid making them composited when they
are `visibility:hidden`. However, if they are composited for other reasons (e.g.
`position:fixed`) then we hide their layer contents, but we fail to hide their
descendant layers, which GraphicsLayer parented via `attachWidgetContentLayers()`.

Fix this by not parenting the widget's content layers in `RenderLayerCompositor::attachWidgetContentLayersIfNecessary()`,
and enhancing the return value to distinguish between "widget has contents via child layers", and "mutated the layer hierarchy"
since the two callers were using the return value in two different ways.

Enhance an existing test to exercise this case.
* LayoutTests/compositing/visibility/iframe-visibility-hidden-expected.html:
* LayoutTests/compositing/visibility/iframe-visibility-hidden.html:
* Source/WebCore/rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::updateConfiguration):
* Source/WebCore/rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateBackingAndHierarchy):
(WebCore::RenderLayerCompositor::attachWidgetContentLayersIfNecessary):
(WebCore::RenderLayerCompositor::attachWidgetContentLayers): Deleted.
* Source/WebCore/rendering/RenderLayerCompositor.h:

Canonical link: https://commits.webkit.org/277643@main

ea4bda8

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-skia
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	🧪 gtk-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 3bcca51)

Details

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	❌ 🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	⏳ 🧪 mac-wk1	✅ 🛠 wpe-skia
	✅ 🧪 api-ios	⏳ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	❌ 🧪 gtk-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash ea4bda8)

Details

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 wpe-skia
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	🧪 gtk-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 277643@main (d927d70): https://commits.webkit.org/277643@main

Reviewed commits have been landed. Closing PR #27306 and removing active labels.