Upon starting a game, I proceed to scroll the screen around and alt-tab a few times.  On my second attempt, I zoomed out, zoomed into the black regions of the map, and alt-tabbed.

When I came back, the screen was black, but the UI was still visible.  After trying to find something, and failing, I clicked on the sovereign icon in the top-left corner, which scrolled my screen back to the sovereign very quickly.

This is what I saw.

I wasn't able to get the exact same thing to reproduce, but I have seen some similar things.  I also have had the game showing only a black screen, along with several other problems.  This behavior is much improved over the last release, but not perfect yet.