Here's a good guess. Tell me your results. Specific-like: hor+? etc. This is the first thing to try. There are more things, if this don't work.
1. In-game, select windowed (it's immediate). Borderless Gaming utility (BG fullscreen setup *). Quit game & BG.
2. Play game: Start BG, start game (auto-triggers full borderless).
NOTES:
1. * Borderless Gaming setup described here:
http://www.wsgf.org/blog/imusrt/2015/03 ... structions2. This for Jotun: Valhalla Edition 2015, a Unity game.
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Failing that...
Next go to regedit. Find (ctrl+F) Jotun (in Keys). Edit resolution (in Decimal Base) to your fullscreen.
While you're there, you could switch back to Fullscreen if you want (maybe a boolean there, 0/1).
Failing that...
If still pillarboxed, you will hex.....