When drawing the possible scrollbar stuff for a nsGfxScrollFrame in nsGfxScrollFrameInner::BuildDisplayList I restricted the dirty rect to the overflow rect for the nsGfxScrollFrame and that fixes the testcase for me. This feels like it is fixing the symptom and not the root cause.
Created attachment 371605
quick fix?
When drawing the possible scrollbar stuff for a nsGfxScrollFrame in nsGfxScrollFram eInner: :BuildDisplayLi st I restricted the dirty rect to the overflow rect for the nsGfxScrollFrame and that fixes the testcase for me. This feels like it is fixing the symptom and not the root cause.