I can't seem to change the resolution - the setup simply ignores what I've set it to and continues to display full screen.

Any ideas how to make it work ?

Thanks!

Greetings. To which game in the Wasteland series might you be referring?

Once that is established, a solution may be reached, as I own all Wasteland games (with the exception of the remastered version of the original Wasteland) on GOG (and play on Linux).

While experimenting with adjusting certain preference values within the appropriate prefs file, I believe that I have found a work-around that allows for the borderless window experience in which you seek (unless I am misinterpreting your overall goal).

Do you have a multi-monitor configuration, or, a single display with which you wish to have the game consume less space (windowed, but, with window decorations removed)? I ask only out of curiosity.

Within "$HOME/.config/unity3d/InxileEntertainment/Wasteland 3/prefs", ensure that the following preference integer values are set:
<pref name="FullScreenMode" type="int">2</pref> -- After reaching the main screen, sets windowed mode.
<pref name="RestrictMouse" type="int">0</pref> -- Allows mouse to be moved outside of window.
<pref name="Screenmanager Fullscreen mode" type="int">1</pref> -- Upon executing game, sets borderless window mode.
<pref name="Screenmanager Resolution Height" type="int">1080</pref> -- Sets initial window pixel height (it is best to use current screen pixel height; modify accordingly).
<pref name="Screenmanager Resolution Width" type="int">1920</pref> -- Sets initial window pixel width (it is best to use current screen pixel width; modify accordingly).

The following two are optional (or, should be completely ignored if you simply wish for a moveable full-screen window), as, after reaching the main menu, I was able to set my preferred resolution (replace 768 and 1024 with your desired values):
<pref name="ResolutionHeight" type="int">768</pref> -- After reaching main screen, sets window pixel height.
<pref name="ResolutionWidth" type="int">1024</pref> -- After reaching main screen, sets window pixel width.

Unfortunately, after exiting Wasteland 3, the Unity game engine will overwrite the values of some preferences (Screen Fullscreen mode, Screenmanager Resolution Height, and Screenmanager Resolution Width). In order to prevent this (and have zero issues upon subsequent launches of the game), it is necessary to restrict write permissions (obviously, this is not ideal, as no other in-game settings changes can be committed to long-term storage), like so:
chmod 444 "$HOME/.config/unity3d/InxileEntertainment/Wasteland 3/prefs"

To undo (when changes are needed):
chmod 644 "$HOME/.config/unity3d/InxileEntertainment/Wasteland 3/prefs"

One of the attached screenshots claims that the screen mode is windowed, but, none of my usual colored window borders are visible. This suggests to me that it is truly borderless, especially due to the inability to manually-resize (without adjusting settings). The behavior of true windowed mode is dissimilar.

If I have misunderstood, I apologize, and I kindly ask for you to state the anticipated window behavior.

why tho

for the record I had no problems playing W3 out of the box, alt-tabbing and minimizing too. Didn't need no special setup whatsoever. Ran the GOG installer and presto.