Patch attached with explanation.
Created attachment 30770 [details]
This seems to break scrolling in the Acid2 test :/
I thought we were always guaranteed to have a scrollbar, even if it would be a dummy one, but apparently in that case WebCore thinks we don't have one and scrolling fails when using ScrollView::scroll. Guess I'm missing something here?
Created attachment 30774 [details]
Be more consistent with our previous implementation, always return true and do not scroll parent views (not sure if that's correct or not). My previous comment still applies though.
Comment on attachment 30774 [details]
Looks good to me. Testing in Chromium/Linux and asking people to test on Safari indicates that not scrolling in acid2 is actually the right thing, so this fixes a bug =).
Landed as r44264, thanks!