I’m not sure what I did, but for some reason my editor window now has a separate background window (yet if I close one, the other disappears too). Does anyone know what I might have done to cause this?
Whatever happened also appears to cause the engine to crash every time I launch in the “Selected viewport / new editor window” modes. Standalone game still works fine, but it takes longer to launch. Please help 
Have you tried clicking and dragging your UEditor window into that background window or vice versa or attemping to add the background thing as a tab in your Editor?
Yep. The non-bugged tabs don’t do anything if I attempt to drag it over the bugged one. The bugged window doesn’t move, but allows me to resize it. Also, if I minimize the bugged window, it minimizes all of my UE4 windows. So I’m currently working like this:
Take a look at this thread: https://forums.unrealengine/showthread.php?33313-Put-the-main-tab-back-in-the-window 
That did it. Thanks!