It seems that when cursor leaves xpra window on client, it remains on it's last position over the window on server. If that position is triggering some popups, they stuck. Cursor on server should also leave window somehow. I.e. place it outside the closest border to the last position.
Sorry, but no. There is no way for us to know when we can safely second guess the client and move the mouse somewhere that the client has not requested.
Maybe, client could request it?
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1222