Can't click top left corner of screen to open menu in gnome-classic

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http://askubuntu.com – Normally you just throw your mouse at the screen corner, click and the menu shows up, right? But in my case I have to move my mouse at least 1px down and right before clicking. Tried with several themes: Ambiance, Radiance, Adwaita. I'm using the "No effects" gnome (meaning no compiz I think). (HowTos)