I am using Chromium (and sometimes Chrome) on 13.04 w/Gnome shell 3.8 with the inbuilt GTK= theme option. In this mode the browser has no title bar. When in this mode, I'm not able to select a tab that's positioned from 0 to ~250px on the horizontal axis. I am able to select tabs that are beyond that range.
If I restore the browser so that it's not maximized and reposition it so that it's 20px on Y axis then I can select tabs.
It seems to be related specifically to Chromium's GTK+ theme. My Gnome shell setup is using the default Adwaita (I tried Faience and the behavior is the same).
I am able to use keyboard shortcuts to select tabs.