~/.steam/steam/steamapps/compatdata/386070/pfx/drive_c/users/steamuser/Local Settings/Application Data/Uber Entertainment/Planetary Annihilation/window_settings.json
Set fullscreen to false (though it seems like it reverted itself, or might not be needed)

Also turn off steam overlay, possibly need to disable desktop game theatre.
I set the process limit to 1 where others were recomending 3, also not sure if that was needed or if my computer is just starchy