This post shows how to get an "off-screen" window back on your Ubuntu 16.04.5 desktop.
Steps
Step #1: A) Click the program group. B) Click the "off-screen" window
Step #2: Press Fn Alt F7 at the same time:
Step #3: Use the cursor keys to move the window until you see it. Using the cursor keys tends to work better than the mouse which may not work.
References
Pic of the keyboard from [link]
Ubuntu logo from [link]
