See testcase. Dragging around the orange div containing the letter "j" will leave trails on the screen.
Created attachment 18712 [details]
Drag and drop the orange box.
Dan suggested this is probably due to the same root cause as bug 12607.
There is no easy and efficient way to tell whether and by how much text will overflow its "typographic bounds", but perhaps WebCore can do a better job at estimating it (e.g. for italics, add a horizontal overflow of sin(italics angle) * (ascent + descent) / 2 on either side).
Myles thinks this is a duplicate of Bug 6274.
*** This bug has been marked as a duplicate of bug 6274 ***