Interesting. I wonder if it was some sort of small patch that was pushed out today.
The last update I can see is from 6 days ago, but the likelihood of us both running into the same issue on the same day around the same time makes me think there was a more centralized event that caused this bug to appear.
I would like to note that I can open individual windows and move them about, but disconnecting tabs from any of them instantly locks mouse cursor input.