Bug 34344 – Adding overflow:hidden to TD elements makes hit testing much slower

• Component: WebCore Misc.
• Status: NEW

[James Robinson]

Created attachment 47733 [details]
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).

[James Robinson]

overflow:hidden causes each TD to get its own layer.  The patch in https://bugs.webkit.org/show_bug.cgi?id=33520 speeds hit testing up

[James Robinson]

Pah, bugzilla was a bit overeager.  What I meant was:

overflow:hidden causes each TD to get its own RenderLayer.  The patch in
https://bugs.webkit.org/show_bug.cgi?id=33520 speeds this test up by about 33%.