I had this bar when I upgraded to Dapper, where i somehow got it to go away. I got this bar again when I upgraded to Edgy and I made it go away by configuring my /etc/X11/xorg.conf correctly according to mouse and graphic card.
-- Edgy 6.10: Mysterious bar under mouse cursor https://launchpad.net/bugs/68447 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
