Commit is a3afa6f2fb80489f7b6a88d12def09281d32ed94 (via git bisect)

I get white rectangles and other rectangular (and usually gray or white) artifacts when moving windows, switching windows, etc.  One notable reproduceable instance of screen corruption occurs when opening Konqueror as a file browser.  It opens in the upper left of the screen.  The white K background, however, extends beyond the right edge of window (leaving the decorations alone, however) to the right edge of the screen, and again from the bottom to the bottom edge of the screen, corrupting the kicker panel.  Such extension does not occur to the left or the top of the window.

I took a look at the code, but since my familiarity with it is still wanting, I wasn't able to find anything blatantly wrong at this time.