For the panels issue, you can launch CS2, press Alt+F3 to open the window settings, go to configure special app settings, then set an option for the app so that it always launches in fullscreen mode.
I also added a shortcut to make games fullscreen by pressing Ctrl+Enter.
I believe this is a bug with KDE and should be reported to them. The same issue is reported on Valve’s repository but nothing has been done from Valve’s side to fix this issue.