I tried your suggestion, to use a fresh/clean browser profile. I read everything on the page at the link you gave me. It gave two ways, one by opening an incognito window using Chrome, and the other to click on something at the upper right and select "New User". I didn't see what to click on in the upper right to be able to select "New User", so I tried the first way. I opened an incognito window, but then didn't know what to do next. It looks like a regular Google page. It says "Google" in big colorful letters in the middle, there's an empty search bar, etc. It said on the instructions for opening an incognito window to "be sure to launch with something that ends with dev null or something like that, and it gives three things for three types of computers. Since mine is Windows, I copied all of that and pasted into the search bar. It led to a page like search results, a lot of Tips and links to useful things for Chrome. I guess I don't know what to do. Can you tell me?

I'm sorry, but I don't know what you mean by "your browser's private mode". Do you mean trying to edit on WP while not logged in? I have never edited WP while not logged in, except maybe once when I first started editing more than three years ago. I'll look at the link you gave me now.

Would it help to do a system restore to about a week ago?

If you tell me what to do on my computer, I can do it if you give me clear instructions.

This has never happened before, and I haven't seen anything like it happen when I'm not logged onto WP. For me to be able to see this, wouldn't I have to be logged on? I never edit without being logged on.

This is the first time I have ever made a screenshot or uploaded anything (except maybe a few images of paintings) to Commons.