[Webkit-unassigned] [Bug 103470] New: Fixed position element under transformed element causes slow scrolling on Chromium

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=103470

           Summary: Fixed position element under transformed element
                    causes slow scrolling on Chromium
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: wangxianzhu at chromium.org
                CC: simon.fraser at apple.com, jamesr at chromium.org,
                    vangelis at chromium.org, klobag at chromium.org,
                    tomhudson at google.com, shawnsingh at chromium.org,
                    skyostil at chromium.org


Any visible fixed position element that is not composited can cause slow scrolling on Chromium (ScrollingCoordinator::mainThreadScrollingReasons -> ScrollingCoordinator::hasVisibleSlowRepaintFixedObjects()).

Fixed position element under transformed element is not composited because of the following code in RenderLayerCompositor:

    // Don't promote fixed position elements that are descendants of transformed elements.
    // They will stay fixed wrt the transformed element rather than the enclosing frame.
    if (container != m_renderView)
        return false;

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