User interface will remain a dumpster fire until we have LLMs (or whatever) helping us. "Computer, stop doing that weird sudden thick blue border thing on my Firefox window, what the fudging heck?!" To wit: suddenly having a thick blue border around you Windows windows. According to the internet, it could be just about anything. Fwiw, in the end in my case it was PowerToys always-on-top keyboard shortcut that was enabled by default and that I did not know about and that I do not actually even want. Turning that off fixed it. (I do actually like PowerToys a lot for the keyboard manager, otherwise.)