This is a platform-native behavior that the old, pre-scrollbar-refactor Chromium code had but the new, refactored code doesn't.
I have a patch for this for both Chromium and Safari/Win (hyatt is not a huge fan of this behavior but prefers it go in both platforms for ease of future code unification).
Created attachment 29560 [details]
Per hyatt on IRC, this implements the functionality for both Chromium and Safari.
Comment on attachment 29560 [details]
Fixed in r42721.