Workspace changed since upgrade.

The image inserted into your post, Redhartotter, is the Home screen that was introduced years/versions ago.

Click the Ps icon in the upper-left corner (red-yellow-green horizontal traffic light), see if that screen is more familiar.

That's the Home Screen, which should go away once you open a document/image.

However, unless you disable it, when no documents are open it will pop back up again.

You can stop the Home Screen from opening and/or popping up by going to Photoshop 2025>Settings>General and unchecking Auto show the Home Screen

Now when you open Photoshop the normal interface should show instead of the Home Screen.

@Redhartotter the easiest method is to just drag that black pane back into the aplication frame and it will be docked

@Redhartotter 

Let's make sure we are using the same terms and definitions. The Workspace is the arrangement of the panels, toolbar, document window and whether the application frame is on.

https://helpx.adobe.com/photoshop/using/workspace-basics.html

The Application frame makes macOS work like Windows and groups all the workspace elements in a single, integrated window that lets you treat the application as a single unit. When you move or resize the Application frame or any of its elements, all the elements within it respond to each other so none overlap. Panels don’t disappear when you switch applications or when you accidentally click out of the application. If you work with two or more applications, you can position each application side by side on the screen or on multiple monitors.

If you prefer the traditional, free-form user interface, you can turn off the Application frame in macOS.

"Now the application frame is floating in the screen, filled with black, yet the menu bar at the top is fixed on my desktop"

I'm not sure what the issue is here. Can you clarify these points?

• Your application frame is off, not on. I think this is just a definition thing, but it's confusing to call it an Application frame when that is turned off.
• I don't see where your window is filled with black? I see the Home Screen with the dark interface.
• On macOS, the menu bar is always fixed at the top of the screen, and changes depending on which application is currently active.

"If I maximise the application frame I can't see the menu bar"

Can you try resizing the window instead of maximizing it? You can drag any corner or edge. 

Jane