I was playing Minecraft in fullscreen on Ubuntu, then I accidentally pressed the super (Windows) key. It opened up the search menu, therefore minimizing Minecraft. However, I could not use the mouse as Minecraft was still open, just invisible. All buttons, and even moving were functional, but the game was invisible and could not be closed or opened back up.
Steps to Reproduce:
1. Start Minecraft on Ubuntu 14.04
2. Go fullscreen.
3. Open a server/world
4. Press the super (Windows) key
5. You will see the regular search interface, but you will notice you cannot click anything and Minecraft is still open in the foreground. Minecraft will be invisible.
Notice: I have confirmed this is a bug with Minecraft, not Ubuntu, by trying this on other programs.