The game wouldn't let me return after being alt-tabbed, so I closed the application and restarted it.  I've done this a few times with no problems.  This time, every single map in the game has a graphical glitch.  The glitch goes away if I hover over an interactive portion of the map, like a character name or a waypoint.   Here are a few screenshots of what this glitch looks like:

 

http://i.imgur.com/aDJq4Bu.jpg

 

http://i.imgur.com/UG0Yi11.jpg

 

I went through steam's "Verify game files", and it finished saying it was successful.  

I have the same issue.

When I alt tab with the map open and then come back to the game, the map image is gone, and there's a texture that looks like stretched wood with strange runes on it.

Closing and reopening the map fixes it.