Currently the caret drawing uses a combination of offsetFromContainer and localToAbsoluteQuad to find out the repaint rectangle of the caret. Use computeRectForRepaint to make sure the filters (and RenderFlowThread for regions) can compute the correct repaint rectangle.
80323 should fix this one because we are catching all the repaints as if that layer was the repaint container. This is similar to how composited layers filter the repaints. I've just tested it and this bug is fixed when the patch on 80323 is applied.
*** This bug has been marked as a duplicate of bug 80323 ***