Comment 8 for bug 1604224

I did a bit of experimenting...

I reliably reproduce the bug by:
Start SMPlayer.
Open a file to play (audio or small video).
Use the window maximize button.
Open another file, that does not require the full screen (audio, or small video)
Whatever was visible on the larger area remains there, the window bar still occupies the full width of the screen, and if one goes back and forth between other desktops and the one containing SMPlayer, whatever part of the screen that SMPlayer is NOT using for its main/Playlist boxes is duplicated from the other desktop.

If I un-maximixe the window, then the effect is not there: if I have a video playing, and open an audio file, it changes to a smaller box, but none of the desktop-duplicating is there.

I will try not maximizing, but merely enlarging the window by pulling on the edges, to see if the effect is there or not...
No. Even if I make the window as large as the screen, it does not exhibit the problem.
But if I take the manually-maximized window and click on the maximize button, THEN the problem manifests as soon as I open a file that does not require the full screen to play.

It seems that the window-maximize button is the key.