If you load a G+ page, such as https://plus.google.com/+ZeeshanAli/posts you will notice that sometimes the scrollbar on the right is styled by the webpage, sometimes it is the native GTK+ scrollbar... and you can trigger this by repeatedly reloading the page until you get the gtk scrollbar instead of the styled scrollbar, as this video demonstrates: https://bugzilla.gnome.org/attachment.cgi?id=307666 Now, I don't actually care that much about the styling, but the thing is that I noticed that the scrollbar sometimes "breaks" and stops responding (bug #143622 ?), and that there seems to be a correlation between that and the "GTK-style scrollbar" being shown, in which case you can't scroll with the mousewheel or clicking on the scrollbar.
G+ is dead and buried, so closing this.