Desktop wallpapers disappear when switch from dual displays to single I started bringing my 2011 MacBook Air to work and hook it up to an external monitor for dual display setup. But when I unplug the display, the desktop wallpapers on the built-in LCD disappears and just show a solid light gray desktop. I have to log out and log back in to get the desktop wallpapers to show again. Is there a way to fix this?

Many apps also still behave as they were on the external monitor. Do I have to use "gather windows" every time I disconnect the external monitor?
I also observed the problem after upgrading my MacBook Air (2011) to Mountain Lion.  Not only unplugging the external display triggers the gray background, but also connecting to a display.

For the gray background problem, I use a quick-and-dirty fix: restart the Dock.
Invoke the following command in the terminal:
$ killall Dock

The Dock will be restarted after it is killed, and the wallpaper will appear again.