Without system information nobody can tell you much. Start by checking the usual suspects - graphics driver, screen configuration, related PS preference settings.
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your problem could be similar to this thread: https://forums.adobe.com/thread/2443449
(mostly written in german language)
There is a checklist you can go through step by step (and the solution in the other thread was: to change the workspace eg to Essentials)
When the window is big enough and if you already work in this workspace try to reset this one (menu: Window --> Workspace -> Reset "your workspace") or change to another workspace.
By the way:
Do you use palettes which are not out of box was installed?
Thank you very much for your looking into this. Indeed, changing the work space to "essentials" has solved the problem. I was using "photography". This never happened before and I hope a fix will come shortly.
I found the solution to my problem though it may not be applicable to all. I moved the tools bar from the right back to the left as per the original layout. Every time I move the toolbar to the right, the same problem occurs.
Hope it helps others
That's not everything what happens. When you choose 'View / Fit to Screen' a window is going to be moved to the bottom!