

The game does not have controller support. Switch controller layout to WASD+Mouse. Use D-pad for movement and "A" as "space".
Text is small but readable
Force proton experemental and change controller layout.
This fixed the all-black (and in 5.0-9, all-white) screen with no interface. Can now see and use full interface and actually play the game.
Windowed only. Using keyboard commands to engage fullscreen did not work, and there were no options for it in the game itself.
The controls locked up on completion of the Hungry Ghost puzzle if the ghosts were interacted with afterwards. The dog was unresponsive and only showed idle animations. Be sure to save before puzzle completion to avoid having to restart the game (only one save slot).
Also tried:
Native Linux version: Launches to NW.JS text animation, nothing else
Proton 5.0-9, no launch options: White screen, music, no interface
Proton 5.0-9, w/ PROTON_USE_WINED3D: Black screen, music, no interface
Proton 4.11-13 w/ PROTON_NO_D3D11: Full interface, music, only tested as far as main menu

Launches with a black screen, music, nothing else
Attempts to run game with Proton 4.11-11, 4.2-9 and 3.16-9 failed; was met with music, a completely black screen, and no menu interface.
This is more than what occurred with launching the native version, which just opened a browser window to nw:blank and a NW.JS text animation.