[Webkit-unassigned] [Bug 261230] New: Clean up overlay scrollbar logic on iOS

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=261230

            Bug ID: 261230
           Summary: Clean up overlay scrollbar logic on iOS
           Product: WebKit
           Version: Safari 16
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Scrolling
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: simon.fraser at apple.com
                CC: simon.fraser at apple.com

iOS has some old #ifdefs, like the one in RenderLayerScrollableArea::showsOverflowControls(), that essentially "turn off" scrollbars in WebCore, since it assumes that they are rendered in the UI process.

However, when canUseCompositedScrolling() returns false, like inside SVG <foreignObject>, then the code still tries to paint scrollbars, which does nothing.

Instead, we should fix ScrollbarThemeIOS to return true from `usesOverlayScrollbars()` and make the iOS code closer to the macOS UI-side compositing code.

Also, at some point, we should support resizes on iOS, which requires that we run code in RenderLayerScrollableArea::paintOverflowControls.

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