The problem was that the title bar and window frames were still off screen or invisible or whatever it is that makes that mode work, so I could not resize the game window - it was still "full screen". And the bottom of the game window was covered by the Windows taskbar - I could just barely see the top of the UI bottom bar above the Windows taskbar, but not enough to click anything in the game bottom bar.
This may not be the best way, but I ended up switching to the default UI, then I could see the frame for the bottom of the game window. I dragged that up, then I was able to resize the game window and drag it so that the title bar was visible again, then I double-clicked the title bar to maximize the game window but leave the Windows taskbar visible. Then I switched back to DarqUI and all is back to normal.