LRC with MAC

Johan is correct, watch the demo below: It starts out like your screen shot, where the middle button is inactive. By choosing Exit Full Screen, you can see that Lightroom Classic exits macOS Full Screen mode, and after that, all three buttons are available.

Why the yellow button is missing in macOS Full Screen mode: You cannot minimize a full screen window, so the minimize button (the middle yellow one) cannot be active. Exiting macOS Full Screen mode lets the yellow button be active again. This works the same way in all macOS applications, not just Lightroom Classic. In the screen shots for the other two applications you showed, Lightroom and Photoshop are not in macOS Full Screen mode, so all three window buttons are active. All of this works as expected.

The other important thing to keep in mind is that Lightroom Classic supports two different full screen modes: The standard macOS full screen mode, and its own. The full screen mode in Lightroom Classic has more features than the one in macOS, and is cross-platform, so it was kept. So it’s important to know which full screen mode you have entered:

• The macOS full screen controls are the green window button, and also the command Window > Enter Full Screen/Exit Full Screen. 
• The Lightroom Classic full screen controls are on the Window > Screen Mode submenu. 

Other notes, which are important if you try this with the other two applications you showed:

• Photoshop still uses its original cross-platform full screen mode, not macOS Full Screen mode. So unlike apps that use macOS Full Screen mode, the green window button only maximizes the window, so to enter the Photoshop full screen mode you choose the command View > Screen Mode > Full Screen Mode (F). Unlike macOS Full Screen mode, in Photoshop full screen mode the menus are not revealed when you hold the pointer against the top edge of the screen. 
• Lightroom uses only the macOS Full Screen mode, because it’s newer than Photoshop or Lightroom Classic.