Show ruler greyed out on lastest projects

On two of my projects the option Format -> Show ruler is greyed out.
Yet on some other projects the ruler is visible (and the option can be toggled).
In both cases I am in document view as usual and I write and edit as usual.

What other option/setting/view renders the option unavailable?

The keyboard focus has to be in the text for the option to be available. If the focus is in the synopsis, or in the binder, or anywhere outside the text, then that option won’t be available because it is tied to the text view.