I noticed with 4.12.4 that when running in editor with PIE with 2 clients the first is maximized with the borders off the screen and the second is smaller. I can move the second window around but cannot Shift + F1 and move or resize the full screen window. Any ideas what happened here?
Turns out that in my case, I screwed it up by having a savegame that got loaded during PIE. This save game had settings pertaining to screen mode + resolution, which were applied as the game started and forced me into a fullscreen bordered window that I couldn’t move around.
For anyone that this is not the case for and is still having issues, here’s a sort of workaround to at least force a size you want: video - How to force windows application to run in 720p resolution and 16:9 ratio - Super User