Comment 31 for bug 946104

After installing and launching Compiz Settings Manager, I chose Ubuntu Unity Plugin under the Desktop section. Click the tab called Experimental. At the bottom, there should be two options: Launcher Monitors and Launcher Capture Mouse. I set the first to Primary Desktop and the second to Launcher Capture Mouse.

It's pretty silly that you have to install compiz just to fix this serious bug. The usability of Unity toolbar leaves a great deal to be desired and its integration should've been postponed until it was more mature.