I've had this intermittent bug for some time (Win 10 x64 currently paint.net 4.216.7781.39227): With several files open, one always a multi-layer file 'cause that's what I'm doing in P.N, when I swap to another window and then return to P.N, the ribbon dialogues are incomplete (eg the red-x box to close a file will not display) or even missing. In the layer dialogue, the layers might be missing or part of the actionable bits or information bits for each is missing. A screenshot from today to illustrate the issue.
In the past, I've had to close P.N and re-open to re-establish all the dialogues. Today I chanced on closing the layer file making the other file's dialogues re-establish. I've yet to see if that was a real work-around, or just a fluke.
In the screen shot, I think I might have already tried to close the layer file (the blue box with nothing in it in the ribbon) but at that point P.N did not seem to have responded.