That should be OK (some distros have started using wxWidgets 3, which we don't fully support).
I've done some more testing on CentOS 6, and cannot reproduce a problem there either. I'm beginning to wonder if it's something more distro-specific, perhaps related to the version of GTK in use, or the window manager. That makes it *far* more difficult to track down unfortunately.
Are you in a position to be able to install the EDB PostgreSQL installer? I'm curious to know if the build of pgAdmin that's included with that works OK. It was built against a fairly old version of GTK for maximum compatibility with different distros.