Comment 12 for bug 327863

I think what's happening is that to fix bug 485030 we stopped clipping the scrollframe dirty rect the scrollframe overflow area. Hidden scrollbars are collapsed so that their size is (0,0) but we don't actually get around to collapsing the scroll buttons and slider children of the scrollbar, so now those children are intersecting the dirty area and being displayed. I'm not sure why this isn't affecting other configurations.

The attached patch is pretty close and would be OK, but it's probably a slightly better fix for nsGfxScrollFrameInner::BuildDisplayList to just check kid->GetStyleVisibility()->mVisible == NS_STYLE_VISIBILITY_VISIBLE.