Lightroom Classic Quits when red button at upper left is clicked.

Lightrom Classic is working like a standard Mac app.

On macOS, if an app uses the multi-window model (like Apple Safari, Apple Keynote, Apple TextEdit, Adobe Photoshop, Microsoft Word…), then clicking the red button does not close the app because there might be other windows. If the last window is closed, the menu bar remains and the application continues to run, in case you want to open a new window.

If a macOS app uses the single-window model (like Apple Photos, Apple iMovie, Apple System Settings (System Preferences), Adobe Lightroom Classic/Lightroom…), if the application window is closed, the application exits, because that’s the only window it uses. If you’re done with that window, macOS figures you’re done with the app and quits it.

No, it isn’t intuitive, but Apple has maintained this odd difference between single-window and multi-window apps for many years, maybe since the introduction of Mac OS X over 20 years ago. If an app quits when you close the last window, then it’s a single-window app that works like Apple Photos and the other well-established Apple apps that also close on the red button.

Apps that use the multi-window model tend to be those that are over two decades old; that style is fading out. Apps introduced more recently are far more likely to be the single-window type that closes on the red button.

(Some might say, but Lightroom Classic has the Secondary Window, so is it not multi-window? No, Secondary Window is set up more like a panel, not a window, so it doesn’t count. For example, Secondary Window hides when you switch applications, like a panel. A window would stay visible.)

Clicking that button closes the catalog. Because Lightroom Classic cannot run without a catalog, it will close the application too. So don't click that button. 🙂