More of a mental note than anything: I have a laptop that I dock and undock. When it’s docked, it has a second monitor. Sometimes I dock, shut down, then start up undocked, which means any applications that had previously been open on the second monitor now open in some nebulous non-space and I can’t do anything with them.

The solution: Activate the application (click on it in the taskbar) and hit Alt-Space-M. Use the arrow keys to move the window to the monitor that actually exists.

(I tried it using the mouse, but the system keeps the mouse in “real monitor space” so it doesn’t work.)