See the attached test case. It appears that when the RenderLayer owning the scrollbars becomes composited we somehow render the translucent scrollbars twice on top of each other or something of the like. I can reproduce the issue in Chromium, WebKit nightlies, and minibrowser. The patch on bug 57202 ensures that this bug doesn't happen when the overflow controls are on their own layer, but it doesn't help when the controls are not.
Created attachment 89537 [details]
This page has a div with overflow:auto with custom scrollbars set to rgba(0, 127, 0, 0.5). The div alternates between being composited and not composited every second. When it's composited, the scrollbar regions appear 'more green' then they should.
Looks like this was cased by https://bugs.webkit.org/show_bug.cgi?id=23740
Created attachment 106935 [details]