REGRESSION (262243@main): LFC enters a broken, semi-enabled state after rendering scrollbars

[whsieh]

337e6be

REGRESSION (262243@main): LFC enters a broken, semi-enabled state after rendering scrollbars
https://bugs.webkit.org/show_bug.cgi?id=255741
rdar://108112443

Reviewed by Simon Fraser and Alan Baradlay.

When GPU process and UI-side compositing on macOS are enabled, 262243@main had inadvertently added a
call to `LocalFrameView::displayView()` in the process of asking for the current page scale; this
instantiates a `m_displayView` in the process, the existence of which causes us to reattach or
detach the root layer in `LocalFrameView::setIsInWindow()`.

`Display::View` was not intended to be used outside of LFC, so we should avoid instantiating it
unless necessary; to fix this bug, we instead add a `deviceScaleFactor()` override hook to
`ScrollableArea`, implement it in all relevant subclasses by asking the `Page`, and then plumbing
this information at paint time to `Scrollbar`, through `ScrollableArea`. This is done (as opposed to
adding a method that reads off of the `FrameView` or `Page` directly in scrollbar) to avoid layering
violations due to accessing non-platform WebCore logic from within `platform/`.

Additionally, we enforce this moving forward by asserting that LFC is enabled when asking for
`Display::View`, which would have fired in the previous test I added in 262243@main.

* Source/WebCore/page/LocalFrameView.cpp:
(WebCore::LocalFrameView::displayView):
(WebCore::LocalFrameView::deviceScaleFactor const):
* Source/WebCore/page/LocalFrameView.h:
* Source/WebCore/platform/ScrollableArea.h:
(WebCore::ScrollableArea::deviceScaleFactor const):
* Source/WebCore/platform/Scrollbar.cpp:
(WebCore::Scrollbar::deviceScaleFactor const):
* Source/WebCore/rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::deviceScaleFactor const):
* Source/WebCore/rendering/RenderLayerScrollableArea.cpp:
(WebCore::RenderLayerScrollableArea::deviceScaleFactor const):
* Source/WebCore/rendering/RenderLayerScrollableArea.h:
* Source/WebCore/rendering/RenderListBox.cpp:
(WebCore::RenderListBox::deviceScaleFactor const):
* Source/WebCore/rendering/RenderListBox.h:

Canonical link: https://commits.webkit.org/263241@main

4b4e531

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	✅ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 15239fa)

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	✅ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	❌ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv	⏳ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash ebad8af)

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	❌ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	❌ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	❌ 🧪 api-gtk
	✅ 🛠 tv	🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 4b4e531)

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	✅ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 263241@main (337e6be): https://commits.webkit.org/263241@main

Reviewed commits have been landed. Closing PR #12987 and removing active labels.