You may be confusing these two things:
- Exiting and restarting Opus.
- Closing the Opus window and opening a new one.
In the latter case, Opus normally stays running in the background. (How to Exit Directory Opus goes into more detail.)
If Opus is exited (e.g. when the machine shuts down) at a point where no windows are open anymore, then the next time it is started there will be no windows to restore. (So Open the listers that were open when the program was last closed won't have anything to open, as there were no windows open when Opus was closed.)
If you run Opus yourself (i.e. not automatically when Windows starts) then Opus will always open something, since the assumption is you are running it to open a window. If there's nothing else to open, it will open the Default Lister.
Long story short:
- You probably want to turn on Preferences / Launching Opus / Default Lister / Update Default Lister automatically when closing a Lister.
Then if you close a window and open a new one, you'll get the same window you last closed.