I upgraded 3 of my computers from Mint 17.2 to 17.3 this week and I noticed all of my already-installed-games in PlayOnLinux won't appear when launching. The game IS running and I can actually see the thumbnail of the game playing in the Panel, but I can't see anything but my Mint Desktop.
Alt+Tab doesn't work
Switching Workspaces doesn't work
If I disable the virtual desktop and set the game to match my Linux Desktop then it works fine, but some games have issues without the virtual desktop. I use it literally for every single game.
Virtual Desktop is still broken in Mint 17.3 Cinnamon
For the last 3 years I normally set it for 1024x768 but now games won't appear even though I hear them running in the background. The "work-around" still works setting the virtual desktop to match your full screen Linux Desktop.