When you click the left "more" arrow in any one of the thumbnail boxes on the right, the thumbnail entering from the left doesn't paint correctly, and you end up with 5 vertical strips, all taken from the right hand side of the thumbnail. The problem is similar to bug 6388 and the fix for that bug should have worked here, too. The reason it doesn't work is that absolutePosition() uses relativePositionOffsetX and relativePositionOffsetY, which access the style, so the absolutePosition() cached before layout is already affected by the modified style.
Created attachment 8908 [details] Reduction Click the Test button, then trigger a repaint (switch to a different tab and back).
Created attachment 8927 [details] Cache layer repaint info right after updating position (WIP)
Created attachment 8934 [details] Cache layer repaint info right after updating position
Comment on attachment 8934 [details] Cache layer repaint info right after updating position r=me. This is a nice one.
Committed revision 14983.