Tested with Proton Experimental (ver. 1 August 2023)

Gaming Mode on Steam Deck

• EPILEPSY WARNING: On the first time you press F4 to go fullscreen after each room transition, both sides of the screen will have strobe flashing until you press F4 again
• Room transitions only happen at intended speed while in F4 fullscreen mode
• Strange graphics appear in the letterboxing mattes when you re-enter F4 fullscreen mode, such as an image of the game before you went fullscreen

Desktop Mode on Steam Deck

• Letterboxing mattes in F4 fullscreen mode show the wrong colour (black) unless you resize the window.
• When windowed, the letterboxing mattes show graphical smearings over the intended background colour (light blue).

Whoops I forgot to mention these (correct as of Proton Experimental ver. 12th January 2023):

• Sound effects cannot play again until they have finished playing (that's why I edited the vine boom to cut abruptly).
• F4 fullscreen view mode is correctly blurred in Desktop mode, but not in Gaming mode.
  • This bug might be intentional in SteamOS Gaming mode. Otherwise, certain GameMaker games with pixel art and/or tiny resolutions (e.g. Undertale, Princess Remedy in a World of Hurt) would end up horribly blurred by the built-in fullscreen mode.
  • Even then, you could change the scaling to Integer/Pixel in the (...) Button -> Power settings menu or just rescale the game window in OBS Studio.
  • F8 scaling mode in Ninja Numpties 1.1.0 was added to take advantage of the F4 fullscreen mode not blurring in Gaming mode, so you could have massive, crystal-sharp pixel art with perfectly square pixels!