@wasimofnazareth wrote:
I've tried all screen options from default to scaled. Think I checked all options in Lightroom.
For the MacBook Pros that have a “notch” for the camera in the middle of the menu bar, there is an option that macOS itself offers. This option is not specific to Lightroom Classic, you will find it available for almost any Mac app.
On the “notch” Macs, macOS now offers an option called “Scale to fit below built-in camera.” You will find this by switching to the Mac Finder desktop, selecting the application icon (not the folder), choosing File > Get Info, and changing the setting “Scale to fit below built-in camera.”

When the option is on, a Mac application will act like it always has on older Mac laptops: It will stay below the camera (notch), so you will always see all of the menu bar. When the option is off, a Mac application will extend up to use all of the screen area available on the M1 Pro/Max MacBook Pro, but then the notch must be visible.
If the option is enabled for Lightroom Classic, you can get the effects you’ve seen: The area appears to change as you go in and out of full screen/Lights Out, and the available screen area seems to shrink or grow as you switch among applications that have that option set differently.
If the option is disabled for Lightroom Classic, it will use as much screen area as it can, but that means you will see a notch when Lightroom Classic is full screen.
Try it both ways and decide whether you like using Lightroom Classic better with the option enabled or disabled.