Out of bounds areas to the top and bottom are drawn due to the steam resolution not being supported. You need to go to the game's page, press the cog, go to properties, go to general, change resolution to 1280*720 and activate the "set resolution for internal and external display"in order to fix this.

By default the controls don't work. Switching to the input "fallen Prince for steam deck, corrected" is mandatory for playing this game. Touch right joystick is the input for interact. Also, all menu screens must be navigated by using the touch screen.