Fix invisible scrollbars in flotaing divs + a bunch of cleanups.
Created attachment 27288 [details] Properly calculate scrollbar location
Created attachment 27289 [details] Avoid needless castings.
Created attachment 27290 [details] Do not hold extra ref to scrollbar
Created attachment 27291 [details] Add manual test for floating div with scrollbar.
Comment on attachment 27290 [details] Do not hold extra ref to scrollbar We can kill the d'tor now. I will do it when landing the patch.
Landed in r40548 - r40551.