'-popupwindow' no longer works?
I switched from using the 'launch arguments' method to KSSL and this argument stopped working.
Strangely, if I check the process details, I do see the command line argument. Looking at how you handle and pass along the arguments, I have no idea how this might not be working correctly, except some tiny feedback that perhaps you should use Join(String, String[]) instead of text = text + " " + str; or such. But that shouldn't be relevant and is more a style choice? I made sure that applies all the way back to Framework 3.0 :D
"../KSP_x64.exe" -popupwindow

I'm using that too, but it seems to work for me. I'm using these command line arguments on the steam launcher:
-popupwindow -adapter 2
I have the same issue, no idea how to fix currently
Hmmm. That is very odd. Do both releases I've made behave the same in this regard?
I'll look into this but honestly, as you said, it should be working. I have very little direction with which to hunt this bug. But I'm looking into it.
EDIT: Wait, are you using "../KSP_x64.exe" as one of your command line parameters? Drop that if so. That'll mess things up. We haven't had to do that since like ancient days.
I assume this wiki's outdated info got you:
https://wiki.kerbalspaceprogram.com/wiki/Startup_parameters
Just list the parameters you want in steam, nothing else.