I have 3 monitors connected to my Windows machine and I have to switch all 3 of them on before logging in or weird things happen. It's only a little bit better on Linux and I don't have a mac to try. I'm actually blaming the modern connectors and how digital inputs are handled, this was never a problem with D-SUB (i.e. you could move the mouse to the empty space where the switched off screen is, but it won't move your icons or your taskbar around...). If there was a global "ignore plug and play for monitors, just assume they are there" I'd use it...
I have an opposite problem - I turn off my monitor and it doesn't want to disappear... before you ask, I switch between inputs because consumer grade KVMs naturally don't support anything more than 1080p@60Hz.
