Thank you, but that did not work, Dave. The image is still zooming but the window is not. (The window adopts a letterbox shape, while the image itself zooms.)
Thanks, Gener7. I tried the Reset. It seemed that the "letterbox-shaped" zoom window was persisting. But while testing things, something happened: Now, windows open to be the size of around one pixel, me having to zoom out by hitting the + key a hundred times. Obviously I've inadvertently set "something" wrong. Any ideas what that might be, with this new clue?
(Dave above talks about checking Resize Windows to Fit. This seems to have no effect anything.)
PS: I've done the Reset Preferences On Quit a second time. Things seemed to work okay, the letterbox effect gone. However, after 10 minutes, things reverted to the "one pixel" size window when opening any window. I didn't inadvertently play with any settings. One minute, files were opening normally, the next, they are opening to pixel size. Bizarre. I wonder if I should do a fresh install of Photoshop. (Though I'm unsure how to Uninstall the current one.)
I wonder about the size of the images you have.
Here is a way to have all newly opened images fit to screen.
Open an image, then open the Action panel, Windows>Action create a new action set, and a new action, name it Fit, for instance, and go the flyout menu of that panel, select Insert Menu Item, then go to the View menu, and select "Fit on Screen"
You can click stop on your action panel.
(this created an action that will maximize a document)
Now, go to File>Scripts>Scripts Events Manager you now need to select the Enable button, and select "Open Document" next to Photoshop event, click on the "Action" radio button, and select your action set, then your Fit action. Click Add, then Done
From now on, each time you open a document, Photoshop will fit it on screen! (I had to create it for a colleague that never zooms in open documents, and the customers have to squint on his screen...)