Only one open window at a time:

On starting up my late 2015 iMac and opening the second of two files, I found that I could have one and only one window open. If I opened a second file, it replaces the first one in the same window, although panes contain the names of both files. Clicking on a name replaces the open file in the window with the file whose name one clicks on. Opening still other files gave each one a pane with its name and the chance to see the file's content by clicking on its pane. But, I can see only one file's content at a time.

I have another standard account on that iMac, and, curiously, in it I can open files in their own windows that stay open when I open still other files. I've run out of ideas about how to fix this problem. Any ideas are welcome.