This isn’t meant to be a complete answer, but just some clarifying comments…
Under View I can't toggle "fit on screen," which may not have changed it anyway.
By @bruceb76700895
Fit on Screen won’t help, because that command’s name is and always has been misleading. It does not fit the window to the screen, in fact it does not fit anything to the screen. What it does is fit the document canvas to the document window. In other words, any time the document window is smaller than full screen, including floating and tiled windows, Fit on Screen fits the canvas to the document window size, not the entire screen size.
Searching with google all I found were outdated instructions, as in "press the green button."
I'm using Windows 11 pro
By @bruceb76700895
That advice won’t help because the “green button” has never existed in Windows. The “green button” refers to how the window Maximize/Full Screen button looks in macOS. The Windows equivalent of that control is the standard, traditional Maximize button at the top right corner of any window, and if you click that on the Photoshop application frame, it actually should fit it to the exact width and height of the screen. So try that, because the Maximize button might solve the problem. (In other words, the “green button” advice is not outdated, on Macs it’s current and can help with this problem…but for Windows the “green button” is not applicable.)
If that does not help, the next question is: Is only the Photoshop application frame not fitting on screen, or is all of Windows 11 (including the taskbar) going off the edge of the screen too? Because if nothing in Windows fits, then maybe the computer display is incorrectly set to something like Overscan.
But there should be nothing wrong with the 1900 x 1200 px screen setting, that’s rather standard.
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