[Webkit-unassigned] [Bug 72078] New: [Chromium] Compositing with full sized fixed div causes heavy repaints

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=72078

           Summary: [Chromium] Compositing with full sized fixed div
                    causes heavy repaints
           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: simonjam at chromium.org
                CC: jchaffraix at webkit.org, jamesr at chromium.org,
                    vangelis at chromium.org, enne at google.com,
                    nduca at chromium.org


Created an attachment (id=114609)
 --> (https://bugs.webkit.org/attachment.cgi?id=114609&action=review)
Reduced test case

This is a reduced case from crosbug.com/21332, which is that techcrunch.com scrolls slowly on ChromeOS. There may be many problems, but at least one of them is that we're repainting every time the user scrolls. One of the troublemakers is a 100%x100% fixed div (#finalizer) that's causing a heavy repaint every time we scroll.

I've attached a simple test case that demonstrates this. If you instrument the code, you'll see that we have to repaint every time you scroll, even though the bright green div isn't visible.

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