1) The version and edition of Linux Mint you are using (if you are not sure, open a terminal and run "mintwelcome").

	Lisa 12 prerelease

2) What you did for the problem to happen, and how to reproduce it.

	Do some operations, like open home directory icon on desktop

3) What happened.

	Upper and lower screen panel disappeared
	Each open window lost its banner,
	no more control on window (resize, move, close)
	No more means to shut down system

4) What you expected to happen instead.

	Panels to stay in place
	Windows to keep their banner

5) If the problem happened once, sometimes, or always.

	Always

Acer travelmate series 800 use Radeon graphics controller.
Everything worked fine in Mint releases < 12.

Also, the cursor arrow sometimes initially shows as a 2cm x 2cm
black and white matrix. In this case, if this square hits
hovering the desktop "home" or "computer" icon, it displays
normal arrow again. (easy workaround !)