Grid rectangle displayed when switching workspaces

[JH]

The following is reproducible 100 % of the time in my desktop machine. It doesn't seem to be specific to any graphic card. It seems to be caused by Unity Launcher (read workaround).

When I switch to another workspace (does not matter if I open any window on it) then return to previous workspaces, if there remains some maximized window and then I unhide unity launcher by any mean (moving the mouse pointer for example) a rectangular transparent "window" appears at the same screen position than Unity launcher, and remains even when Unity hides. See attached image.

In resume: open any window, maximize it, switch workspace, come back to first workspace, show Unity by moving the mouse pointer to screen side, the "rectangle" will appears.

Workaround: One option is to restart the session. Other is to restore the maximized window and move it from left to right for a while, this will cause launcher to unhide and hide again and the rectangular "window" will disappear. Note: changing how launcher hides does not solve anything, the rectangular "thing" is still there when I switch workspaces as long as a maximized window is in the first workspace, probably if one maximized window is in the second workspace the same will happen but in the second workspace.

Ubuntu version: 11.10 (AMD64)
Session type: default
GPU: Intel GMA X4500

All updates to date applied.

See image.