If smooth scrolling is enabled or a scroll wheel is used and an element changes style during scrolling, it is not always repainted completely. To reproduce: 1. In System Preferences > Appearance, select "Use smooth scrolling" 2. Open the test case 3. Scroll to the middle of the page and place the mouse pointer over the square 4. Click once to give keyboard focus to the frame 5. Use the up and down arrow keys to scroll the square out from under the mouse and back in. You can also try using the scroll wheel to do that Expected result: The square should be painted with one solid color at all times, red when it is under the mouse and green otherwise. Actual results: Occasionally the square is part green and part red. Regression: I couldn't reproduce in shipping Safari with this particular test case.
Created attachment 11573 [details] Test case
<rdar://problem/4960252>
Fixed by <http://trac.webkit.org/projects/webkit/changeset/18704> (remove window display throttling code; no longer used).