Comment by wtallis

> Meanwhile my Mac somehow guesses which side of the computer the monitor is on (even on new setups) almost always correctly. That last part I have no idea how they do it.

I have no idea how macOS does it, but the obvious thing to try is to leave the relative positioning undefined until the first time the user tries to move the mouse off one screen, and assume they're aiming for the other screen.

It would probably make sense to constrain this to horizontal movements, so that taking advantage of Fitts' Law to hit the menu bar or the Dock (at the bottom by default) wouldn't produce a false positive signal about display positioning when stacked display setups are less common than side by side.