I've got my iMac connected to my MacBook Pro as an external display (for now). It works beautifully, at least until I shut down / sleep the laptop, at which point it seems to forget that it was in external display mode.
I have a wireless keyboard, so that means I have to re-pair it with the monitor iMac so I can CMD-F2 it back to target display mode. Really annoying. How can I make it a "sticky" setting, or at bare minimum, put it back into target display mode without having to connect a keyboard again?