Shouldn't this be updated too with the new line?

Ideally yes but I haven't figured out how to express this with Pp. The algorithm that Pp uses for formatting breaks lines in such a way that it breaks between Hint: and the message depending on the length of the message. In this case it splits if the concatenation of all depext names is longer than some threshold even though the depext names are in a vbox so their total width shouldn't matter (only their maximum width).

It might be a bug in Pp though I don't completely understand Pp.

Shouldn't this be updated too with the new line?

This should be ~f:Fun.id and you should put Pp.hovbox on the first line and Pp.vbox on the second. Then you should do a Pp.vbox after concatenating. I think this will give you what you want.

I assume you mean:

let depexts_hint = function
  | [] -> None
  | depexts ->
    [ Pp.hovbox @@ Pp.textf "You may want to verify the following depexts are installed:"
    ; Pp.vbox @@ Pp.enumerate ~f:Pp.verbatim depexts
    ]
    |> Pp.concat_map ~sep:Pp.cut ~f:Fun.id
    |> Pp.vbox
    |> Option.some
;;

I tried that and it had the same behaviour as the current state of this PR.