How do you force OS X to re-detect displays?
Go into System Preferences->Displays and when there (doesn't matter which tab) press the 'Option/ALT' key and you'll see 'Detect Displays' where the 'Gather Windows' button is (on 10.13.x) or simply in the lower right on 10.14.x (as this page doesn't have a 'Gather Windows' button.)