Okay, something of a “first world” problem here , but I just replaced the crappy 1400x900 3rd monitor I was using with a 2560x1440 monitor, and made it my primary, instead. My other monitors are both 1920x1080. Now, I’m finding that when I start a VASSAL module, the controls window opens on monitor #1 (which is at coordinates [0,0]), but is 2560 pixels wide (and so stretches onto monitor #0, at [1920,0]). Monitor #2 (the new one, now designated as primary), is located “below” #1, at coordinates [0,1080].
So, it looks like VASSAL is using the dimensions of the primary monitor to decide the initial size of the controls window, but placing it at coordinates [0,0], even if that isn’t the primary monitor.
I’m running on Linux (Kubuntu 20.10), FYI.