Go to a page with a vertical scrollbar (I use news.google.com). Click once in the track just below the thumb. Note that only half of the thumb is drawn hot.
This regressed some time before February 20, 2009 (that was the date I filed this for Chromium).
I have a fix for this locally.
Created attachment 29540 [details]
Because we don't draw a hovered state if there's a pressed part, we need to make sure we invalidate any hovered part when we reset the pressed part, so that it can begin drawing its hovered state correctly.
(The "half the thumb is hot" effect from before was due to the way the track and thumb rects overlap, since we were invalidating a track piece.)
Fixed in r42624.