[webkit-changes] [WebKit/WebKit] 337e6b: REGRESSION (262243 at main): LFC enters a broken, sem...

[Wenson Hsieh]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: 337e6bee8615c22c0fdd27f6af7481a16af57182
      https://github.com/WebKit/WebKit/commit/337e6bee8615c22c0fdd27f6af7481a16af57182
  Author: Wenson Hsieh <wenson_hsieh at apple.com>
  Date:   2023-04-21 (Fri, 21 Apr 2023)

  Changed paths:
    M Source/WebCore/page/LocalFrameView.cpp
    M Source/WebCore/page/LocalFrameView.h
    M Source/WebCore/platform/ScrollableArea.h
    M Source/WebCore/platform/Scrollbar.cpp
    M Source/WebCore/rendering/RenderLayerCompositor.cpp
    M Source/WebCore/rendering/RenderLayerScrollableArea.cpp
    M Source/WebCore/rendering/RenderLayerScrollableArea.h
    M Source/WebCore/rendering/RenderListBox.cpp
    M Source/WebCore/rendering/RenderListBox.h

  Log Message:
  -----------
  REGRESSION (262243 at 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 at 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 at 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