Despite testing thoroughly on Windows, Mac, and Linux, I didn't realize the game was unplayable on Steam Deck. It couldn't detect the correct screen size, so only the top left corner was visible.
After finally getting my hands on a Deck, I discovered the native Linux build was causing the issue. Strangely, the Windows version via Proton worked fine.
It took a lot of investigation and trial-and-error, but I eventually worked out a solution that makes the native Linux version run properly on Steam Deck, while still behaving as expected on desktop platforms.
If you previously launched Dragon Riddler on Deck, your settings might still force the wrong resolution. To reset them, tap the top left corner of the main menu and press the green confirmation button.
This hotfix should install automatically, so you don't need to configure anything.
Please let me know if you find any bugs!
Cheers,
Sandor