If the document includes an iframe that has scrollable content in it, and the iframe has a rule that makes it's body element's background a color (e.g. body { background: #ffffff; } *and* the iframe has something that floats over it (e.g. think popup menu on the main document ), when the iframe is scrolled, the floating element's pixels are scattered all over the underlying iframe. will attach a small test-case that illustrates the issue.
Created attachment 17690 [details] promised attachment.....
*** This bug has been marked as a duplicate of 5909 ***