My main monitor (as defined by display settings) is the bottom one so that the taskbar is there.
The monitor I like playing games on is the top one.
When I launch Factorio I have this silly workaround where:
- I press Alt+Enter and wait for the startup process to take my keystroke. It usually takes one or two seconds.
- I drag the window up to the correct monitor. No delay.
- I press Alt+Enter to return to full screen. It takes a short while to process that keystroke again.