I've found the bug and reported the fix upstream. Implicit pointer grab release was broken in GTK+2. It was not releasing implicit pointer grabs when modifiers like NumLock were active at the moment the button was released. The behaviour described in this bug was a side effect of this. You can see this by trying to reproduce without numlock on.
** Attachment added: "patch fixing implicit grab release with numlock on" http://librarian.launchpad.net/6069288/010_implicit_grab_release.patch -- Resizing columns need focusing first https://launchpad.net/bugs/62898 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
