This problem started about two days ago right after I made an edit to my own talk page in the "Bristol" section. (A day before that, I had trouble with copy-and-paste editing in the Bristol article; I don't know if these two issues are related or not.) See Wikpedia Village Pump (technical) "Having trouble making edits". For the last two or three days I have been unable to edit using Chrome. I can read articles and comments. When I click "Edit source" to make a comment or edit, at first the edit window looks normal but it quickly changes, becoming a narrow band running from the far left of my screen to the far right. The menu that is normally along the left side of the screen appears under, or through, or in, the edit window, as does the Wiki-markup at the bottom of the page. I can type in the edit window, but when I click "Save", I get the box that says, "Do you want to leave/stay on this page?" I cannot save an edit. I tried disabling Visual Editor (which I only enabled about a month ago and have not used). That didn't work. I tried uninstalling Chrome and then reinstalling it, and that didn't make any difference. I tried re-setting all my preferences to the original default settings. That didn't work. I was unable even to ask for help at Village Pump technical because I couldn't save any edits. I installed Firefox and have been editing from Firefox without any problems. I think I prefer Chrome because it's a bit faster than Firefox. Is there anything you can do to help?
This is the first time I have ever made a screenshot or uploaded anything (except maybe a few images of paintings) to Commons.
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.
Regarding the other things, I don't even know what browser add-ons or browser extensions are, and I don't know what a fresh browser profile means.
Does it also happen when you are not logged into Wikipedia (e.g. via your browser's private mode)?
Have you tried using a fresh/clean browser profile? See https://developer.chrome.com/devtools/docs/clean-testing-environment
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.
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?