[Webkit-unassigned] [Bug 33408] New: Add flag IGNORE_FIXED_BACKGROUNDS to ignore fixed background images and accelerate web page scrolling

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=33408

           Summary: Add flag IGNORE_FIXED_BACKGROUNDS to ignore fixed
                    background images and accelerate web page scrolling
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore Misc.
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: dbates at webkit.org
                CC: ddkilzer at webkit.org, treat at kde.org


Proposed optimization to ignore fixed background images (i.e.
background-attachment: fixed) when a page does not contain any fixed position
elements so as to allow fast repaints (via bit blit) when scrolling a page.

On low-powered/mobile devices slow repaints can cause noticeable delays when
scrolling a page with a fixed background image. Currently, if a page has
elements that specify either a fixed background or a fixed position then we
perform a slow repaint (i.e disable blitting) so as to avoid rendering
artifacts. By sacrificing support for fixed background images when there are no
fixed elements on the page and with come care, we don't need to force slow
repaints and can avoid rendering artifacts. Hence, on such devices we can
improve the responsiveness of scrolling a page with a fixed background image.

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