On any frameset page, dragging the frame resizers makes them disappear or be replaced with junk.
Must have been caused by removing repaintObjectsBeforeLayout() :-(
Created attachment 14621 [details] RepaintDuringLayoutIfNeeded Actually wrote a patch that carefully repainted only the resizers before I realized it was pointless :)
Comment on attachment 14621 [details] RepaintDuringLayoutIfNeeded Heh. r=me.
Landed in r21603.