A workaround exists by disabling a specific plugin(s) that can cause the issue, try prioritizing the ones using dock UIs. Another one would be dragging the viewport out and back into its place although the problem will persist.
By any chance do you use the advanced camera plugin, because from what I have seen that plugin causes that exact error, I even created a modification of it because of the bug. It creates a window on that exact space and when you hit the advanced camera settings it fills in that widget, hitting the settings button again closes the widget.
I specifically ask this because the box is the exact size the advanced camera settings widget is.
Yea I figured, well the only 2 fixes are to sort through your plugins and find out what causes the stray window to appear or just undock then redock a window, sorry I couldn’t help. (I actually think I had like 3 plugins that caused it…)
Oh, well actually I do have some help, you could click through every one of your enabled plugins and see which ones make a window in the same position and size, then disable them and see if that fixes it. Instead of going through each disabling and reenabling them individually (I just said what the 1st guy said… )