[Webkit-unassigned] [Bug 75001] New: Painting on a table with overflow: hidden cells is very slow

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=75001

           Summary: Painting on a table with overflow: hidden cells is
                    very slow
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
               URL: http://dglazkov.github.com/performance-tests/biggrid.h
                    tml
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: jchaffraix at webkit.org
                CC: dglazkov at chromium.org, jamesr at chromium.org
            Blocks: 73714


Seen on the benchmark: http://dglazkov.github.com/performance-tests/biggrid.html. If you toggle overflow: hidden and scroll on the table, you can see that painting takes several times more time that without overflow: hidden (for example by looking at the WebInspector timeline).

RenderTableSection implements an optimization during painting where it finds the dirtied cells to paint using a binary search on the rows and columns (see RenderTableSection::paintObject). When we toggle overflow: hidden, the painting goes through the RenderLayer path. However there is no such optimization and we spend a lot of time trying to repaint layers that are not dirty (or even worse off-screen). This bug may be related to bug 73715 but it looks like there is more to the other bug (composition is involved for example).

As a side note, hit testing has exactly the same issue.

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