Twice now in different online sessions, I have been checking inventory and selling various things I didn't need, and, with the vendor window open, I open my stash tab and it correctly swaps windows. Then, I cannot do anything with the items in my inventory. They are visible, but not responsive. If I click anywhere else, that window never goes away. It's like a UI remnant that cannot be removed from the screen. Logging to the Main Menu and back is the only thing that seems to fix it.

I just repeated it again. Move so you can see your stash tab while the vendor window is open, then open your stash tab. Bingo.