[Webkit-unassigned] [Bug 34344] New: Adding overflow:hidden to TD elements makes hit testing much slower

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=34344

           Summary: Adding overflow:hidden to TD elements makes hit
                    testing much slower
           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: jamesr at chromium.org
                CC: simon.fraser at apple.com


Created an attachment (id=47733)
 --> (https://bugs.webkit.org/attachment.cgi?id=47733)
Test page using elementFromPoint

Setting overflow:hidden on <td> elements inside a large table increases the
amount of time needed to hit test points within the table.  This is just a
special case of https://bugs.webkit.org/show_bug.cgi?id=33520 - setting
overflow:hidden gives each <td> its own RenderLayer, so hit testing within the
element causes a lot of very slow coordinate maps.  The other issue is that
currently when hit testing inside a table we iterate through every child cell
instead of taking advantage of rows to restrict the search.

This shows up as a 30-50ms delay on an internal app with a large table embedded
at a moderate depth inside the DOM (roughly 10 RenderLayers from the document
root).

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