The title says it all.
And as it doesn’t accept custom values, there is no chance to type something in manually.
Edit:
It seems that it works and custom values are supported, but the UI is just broken (value doesn’t transfer from dropdown to input when clicking).
I swear after some fiddling around, the main tabs received list styling, when applying it to the li tag inside the Globals:
So that we keep track of things in relation to the post on the Facebook group about body global elements being applied to the inspector elements, moving to Bugs.
Didn’t pay enough attention to this in the first place, thanks @Marius.