Mac screen sharing not working — the commonest causes

The others only see a black screenThe application really is sharing — it is just not allowed to read what is on the screen.
The application is not in the list at allmacOS only lists an application after it has asked for the permission at least once.
The switch will not moveTwo completely different causes that look identical.
I allowed it and it still does not workAlmost always the same thing: macOS does not apply a freshly granted permission to an application that is already running.
I can see the screen but the pointer does not moveSeeing and controlling are two separate permissions on a Mac, and this is the commonest confusion there is.
Only one window is visible, not the whole screenA single window was chosen when sharing instead of the whole screen.
The second monitor is not being sharedDisplays are shared one at a time.
Nothing can be changed on a company MacManaged Macs have their permissions set by IT.