Hi there.
Thanks for releasing this game on GOG and for Linux.

System: officially unsupported up-to-date Arch Linux.

I just started playing and have a few minor bugs to report.

1. The sword cursor flickers when hovered over interactive objects. I think it's supposed to just 'glow'. Weirdly enough this seems to be less of an issue when the game doesn't have focus.
2. On the card detail screen the left rather then the right mouse button is indicated, see screenshot (massively reduced due to filesize limit).
3. This may not be a bug or a art bug, but I have the impression that the characters in dialog don't look as good as they could. Maybe it's just the way the original art is drawn, maybe the original wasn't drawn large enough, but I feel in this rather big version it doesn't look quite as good as it could. It's pretty noticeable on Copernica.

Another minor bug:
If you play less than three cards, for example because you only have skill cards that you can't afford on your hand, then you can't end the turn using just the mouse. Not a big deal to press 'Tab', but the rest of the game is controllable by mouse only.

What is it that Ubuntu does that makes fullscreen mode worse and worse for the likes of me? I sue FVWM as my window manager. Some time in the last two years, games started not disabling window decorations when going full-screen. Nearly 20% of my games now require explicit window manager overrides to keep the decorations off. Around the same time, some games also iconify themselves when going full-screen. These also all fail to remove window decorations, so half of the above 20% now do this. This game takes it a step further: not only does it iconify itself, it refuses to size itself to my screen resolution, requiring a manual edit of .local/share/SteamWorld Quest/settings.xml. Unless I missed that configuration in the GUI. Actually, it even iconifies itself when not in full-screen mode

Of course I don't expect anyone to fix this. I just want to know what Ubuntu/Gnome does that makes devs feel the need to do these things. How does iconifying the window even make any sense at all?

I can't answer any of that unfortunately, but I can tell you how it behaves on my system (Arch Linux, i3 WM). A lot of games behave like this one.
With the game set to Fullscreen it starts fullscreen. If it loses focus it switches to window mode, which isn't visible in game but it's in a window with decorations and all. When I then use the window managers shortcut to force it to fullscreen it does just that and remains in fullscreen even after losing focus.
For me that works fine since I'm in control via the WM, it doesn't matter whether it's set to fullscreen or windowed in game, it's only initially a bit odd as it goes from fullscreen to windowed when losing focus.

Actually, I think you identified why 10% of my games iconify themselves. FVWM has always been a bit weird in assigning focus, and it probably accidentally doesn't give the games focus at startup, causing them to iconify themselves. I guess they think it's a feature, not a bug. Oh well. At least I can explicitly disable iconification if they do such a stupid thing, but I shouldn't have to edit my FVWM configuration every time I get a new game. Maybe I'll turn it around and only allow iconification for specific, known, well-behaved apps.

Edit:
On the other hand, these games that iconify themselves also refuse to allow themselves to be uniconified. I suppose it might still be a focus issue, and maybe a bug to take up with the FVWM developers. I rarely iconify, though, and I think just disallowing iconification for all windows is the only answer I'm motivated enough to use. Let someone else deal with it. Still doesn't solve the issue of window decorations still appearing, and of course the specific issue with this game guessing the wrong resolution and not having any in-game method of fixing that.