Surprisingly straightforwardly - when you open the lid, focus is initially still on the desktop, not on the Lock Screen. You may have noticed if you’re too quick with your password the first few characters don’t appear. They don’t go nowhere - they go to the desktop.
If you get a certain key combination in before focus switches, it stays on the desktop, and you can continue to input - fire up a terminal, do whatever you fancy. It’s all blind, but still perfectly dangerous.
Certain full screen apps, if they have focus when you lock, retain focus indefinitely. Paradox interactive games, for instance - stellaris exhibits the behaviour nicely. This even includes mouse focus, and if you Apple-tab, then focus goes to whatever you Apple-tab to. You can only restore focus to the Lock Screen by clicking in the password field.
Both times I reported this I got a pedantic “locking the screen does not terminate applications, which may continue to run in the background” response.
