[Webkit-unassigned] [Bug 99669] New: hover, layout and scrolling super slow

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=99669

           Summary: hover, layout and scrolling super slow
           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: ojan at chromium.org
                CC: eric at webkit.org, esprehn at chromium.org,
                    jchaffraix at webkit.org, simon.fraser at apple.com,
                    jamesr at chromium.org, enne at google.com,
                    eae at chromium.org, leviw at chromium.org, pdr at google.com


Created an attachment (id=169322)
 --> (https://bugs.webkit.org/attachment.cgi?id=169322&action=review)
test case

Here's a reduced test case from a Google app. It's actually pretty close to the original in terms of performance. I tried reducing it more, but it was hard to tell if I was removing important bits. There are at least a few bugs here (probably more):

1. The hover takes forever to kick in. This gets worse as you get lower on the page. Clearly part of this is that we iterate over every renderlayer until we find the one we need to hit. Should we be maintaining a quad-tree for the positions of RenderLayers so we can make this be O(log n) instead of O(n)?
2. Mousemove events fire multiple times while scrolling. Firefox only fires one mousemove at the end of the scroll animation. We clearly should do the same.
3. Probably due to both of the above, scrolling is super janky.

If you remove the fixed position div and have threaded composited scrolling enabled, then scrolling becomes very smooth, but you see a lot of checkerboard and it's now differently janky. :(

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