[Webkit-unassigned] [Bug 86651] New: FrameView::scrollContentsFastPath should use painted area to determine whether to drop out of the fast path

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=86651

           Summary: FrameView::scrollContentsFastPath should use painted
                    area to determine whether to drop out of the fast path
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Keywords: InRadar
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: timothy_horton at apple.com


<rdar://problem/11459243>

FrameView::scrollContentsFastPath has a fixedObjectThreshold of 5: if there are more than 5 fixed-positioning elements on the page, it will fall out of the fast (blit) path and repaint everything.

This doesn't make sense - causing repaints of the whole page just because someone used 5 (or 10, or whatever) tiny fixed-position elements is wasteful (and vice versa, blitting the whole window and then just going ahead and redrawing it all because someone used only one enormous fixed-position element also doesn't make sense).

We can make trivial use of regions and threshold based on percentage of the view that will need to be repainted by fixed-position elements.

I've tested a few different thresholds with an internal test; 50% seems to work relatively well, but the ideal value is hard to determine and likely depends on hardware.

I have a patch.

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