0) I've been playing version 1.4.1 of the game, if all else fails, maybe try downloading that earlier version?
1) I have it set to 1920x1080, and I have the "Set resolution fo internal and external displays" option checked. I don't even see a full-screen option in-game?
2) I've just been playing by mapping the steam deck controls to keyboard + mouse. There was no gamepad support for the version of the game I was playing.
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So I downloaded the latest version to play with it. Looks like gamepad support is, indeed, totally borked. Left stick down works, left stick right is left, and up and left do nothing at all, haha. Restarted the game, disabled gamepad input. Controls as follows:
Set dpad/left stick to WASD for movement
Start = E for edit mode
Select = Escape for map
A = space for jump
B button = R for restart
Right stick = mouse, R2 for left click
Just gotta' use keyboard + mosue control mapping and it seems to work fine still. And try unchecking the full screen box. You might have to launch the game in desktop mode if it's off-screen. Good luck friend!