It might be more useful to provide exact technical info instead of ominously speaking about "external display" and so on. to be blunt, based on your post we know squat, which makes it impossible to assist even based on a crude guess.
Moderator please delete, then.
1 person found this helpful
It could be a graphics driver issue.
I have a similar issue with Full Screen Mode With Menus using external display.
Cintiq 22" (Non Touch) Pen Display
MAC OS Sierra 10.12.1
Using Adobe CC 2018 19.1.0
WHEN I GO TO FULL SCREEN MODE (With Menus) the canvas is aligned to the upper left, when it is supposed to be automatically centered. Recentering isn't possible unless I got to Full Screen Mode without Menu. It's disorienting and a little hard to explain. Image attached. The only other thing I can tell you is that it's a CC 2018 update. Never had this issue before. Haven't changed my OS in a year.