OpenJDK 21 uses the improved GTK Look’n’Feel. OpenJDK 17 is the latest, I know of, that has more compact layout and uses an older GTK integration. That’s what the two screenshots above was meant to illustrate:
- On the left-hand side is shown Vassal run by JRE 17, with compact layout, and
- on the right-hand side Vassal run by JRE 21 which has a more “exploded” layout.
Whether you run Vassal as “single window” or “detached windows” doesn’t make a difference on how the Look’n’Feel sets up the UI.
Whoops, yes you are right - my mistake. When you do as outlined in Set VASSAL Look’n’Feel - i.e.,
$ VASSAL.sh -Dswing.systemlaf=...
it will only change the Look’n’Feel for the Module Manager. I got confused because there’s some development that addresses this issue, and because I used the --direct option, as in
$ VASSAL.sh --direct -Dswing.systemlaf=... --load MyModule.vmod
which by-passes the Module Manager and executes the Player with the given module directly.
The _JAVA_OPTIONS method of setting the Look’n’Feel does work though. I’ve fixed this in Set VASSAL Look’n’Feel .
One could argue, that is is up the module developers to not overload the toolbar with buttons, and perhaps organise them into menus. A toolbar with much more than 20 buttons can be hard to get an overview of, and it may give a more intuitive and clear UI if these buttons where organised into thematic menus. UI design isn’t always easy 
Yours,
Christian