Apologies if this topic has been covered . . . didn't see any reference to it in my search. Since the big CL7 upgrade any pop-up window, whether a notification or pop-up window launched with a link-click, causes the pop-up window itself and any subsequent dropdown windows and Nautilus file-manager windows to go black. Logging out and back in rather than a complete reboot solves the problem - until the next notification pops up. I am running LMDE 64 on a Dell Optiplex 755 with an Intel Core 2 Duo processor and Intel GMA 3100 graphics chipset. I run updated LMDE 64 on an AMD 64-powered desktop at home and have experienced no problems with the same upgrade.