Visit http://www.iquilezles.org/apps/shadertoy/ with WebGL enabled, and try typing into Source text box. Note that your typing doesn't show correctly (but you do see some weird pixel dust).
Created attachment 45414 [details]
This is related to table redraw in compositing mode, but, oddly, even when the table is not in a compositing layer.
This is related to the code that walks up the container() chain in computeRectForRepaint(). RenderTableRow and RenderTableSection are both adding offsets?
Hmm… perhaps something needs to be done to compensate for <http://trac.webkit.org/changeset/51864>? Or just more of the same?
Actually RenderTableCell::computeRectForRepaint() appears to be doing the right thing. Are we starting the computation at the table row?
Ah, we're getting a difference answer when RenderBox::computeRectForRepaint() goes through the LayoutState code path.
Created attachment 45586 [details]
I wasn't able to create a repaint layoutTest that shows the problem, so no test here, alas.