Compatibility Report

The game is built using the open source Adventure Game Studio v3.3. Luckily for us it logs a lot of info using OutputDebugString*().

The game uses 320x240 native resolution. You see "Resolution: Auto-Detect" because of the graphics filter - it does the 2-8x nearest-neighbor scaling. Set the filter to 8x and it will say "Resolution: 2560x1920".

The engine automatically finds the best scaling using the current mode (3840x2160). It settles on maximum 8x in your case (2560x1920). It then tries to find a resolutions with the exact height of 1920, and the closest possible width to 2560 allowing some leeway on the sides for pillarboxing.

It doesn't find any mode with the 1920 height, tries to fallback to 320x240 and few other resolutions but ultimately it fails.

In Proton 5.0 there are two sources where it gets list of resolutions from:
a) XRandR
b) "fullscreen hack"

We can discard "b", as it's generic and doesn't contain any mode with 1920 height. "a" should be coming from your hardware - the EDID of your monitor.

Now we have 3 options:

1. your monitor doesn't support any mode with 1920 height - the failure would also be present on Windows and we cannot do much about it
2. xrandr doesn't expose that mode - that may happen e.g. if the mode gets pruned from the list or you run Wayland - XWayland usually exposes only the current mode through xrandr
3. there's a bug in Wine/Proton and we don't add the resolution to the list - I haven't noticed anything suspicious in the code though

@omicron-b Can you provide me with output of xrandr (cmd line utility) and more details on what desktop environment / window manager you are using?

@ivyl looks like option number 1 to me
I am running GNOME 3.36 and X11

Screen 0: minimum 8 x 8, current 3840 x 2160, maximum 32767 x 32767
DVI-D-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
   3840x2160     60.00*+  30.00  
   2560x1440     59.95  
   1920x1080     60.00    59.94  
   1600x900      60.00  
   1280x1024     60.02  
   1280x800      59.81  
   1280x720      60.00    59.94  
   1152x864      59.96  
   1024x768      60.00  
   800x600       60.32  
   720x480       59.94  
   640x480       59.94    59.93  
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
DP-4 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)

@omicron-b indeed it's option 1. This problem most likely would be present on Windows with that monitor. Thanks for the confirmation!

I think it may be time to add a bunch of common <= 4K resolutions to fshack to make life easier with quirky games like this, especially that Wayland becomes more popular, but that's a separate thing.

Confirmed it's the same error on Windows as well with a 4K screen. This needs to be fixed on the AGS side.

AGS seems to have that fixed since 3.3.2 (September 2014). The resolution selection code there is much more robust and it supports both letter- and pillarboxing.