[Webkit-unassigned] [Bug 92011] New: 50% fixed position coverage slow scroll heuristic is incorrect when invalidations aren't clipped

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=92011

           Summary: 50% fixed position coverage slow scroll heuristic is
                    incorrect when invalidations aren't clipped
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore Misc.
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: enne at google.com
                CC: simon.fraser at apple.com, timothy_horton at apple.com,
                    vollick at chromium.org


The heuristic in FrameView::scrollContentsFastPath that tries to estimate the coverage of fixed position elements on the screen is incorrect if !FrameView::clipsRepaints().  This is because you could have a very large off-screen fixed position element that covers more than the viewport size.

Some thoughts:
(1) Maybe fixed position elements should be explicitly clipped to the viewport or to some ancestor clipping layer of their container.

(2) For Chromium, this heuristic is not useful at all when the view's layer is composited.  We don't shift scroll composited layers, so the slow scroll "invalidate everything" path is always going to be slower.  Maybe we could just skip this heuristic for composited layers?

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