[Crash] Screen goes completely black. All I can see is the UI, map, and chat

Not sure if this is a bug or not, but it has happened twice in the last 2 days. I don't have the first time recorded, but this is the second one. It only happened after I was trying to Alt+Tab while the loading screen was loading. I used to be able to Alt+Tab even before the loading screen loaded, but now I can't. It just kind of glitches out. I don't know if this is my computer, or just my client, but I have also been lagging much more than before lately, and sometimes it gets pretty bad.