If you grab a slider's thumb and drag it fast enough (on a machine that's slow enough) past the edge of the slider, it doesn't move all the way to the edge, and when you release the mouse (still out of the slider's range), the thumb maintains its "pressed" appearance (this is true whenever you release the thumb with while the mouse is "out of range"). Forcing a repaint fixes thumb position and appearance. Another bug that is probably related is that when you click the slider track and hold still, the thumb gets the "pressed" appearance but doesn't jump to where you clicked. Thumb painting seems to be always "lagging by one event".
<rdar://problem/4960133>
Created attachment 13101 [details] Update the thumb's layer's position
Created attachment 13102 [details] Automated "manual" test This test works in Safari but not in DumpRenderTree.
Comment on attachment 13101 [details] Update the thumb's layer's position r=me
Committed revision 19556. Added the automated "manual" test to WebCore/manual-tests.