StaffSymbol #' staff-space definition produces a SystemStartBar (?)

[Valentin Villenave]

Greetings everyone,

On my way to answer Adam's recent mail, I found something really surprising:

why does

\new Staff \with {
  \override StaffSymbol #' staff-space  = #1.4
} {a b c' d'}

produces a single-staff system with a left barline at its beginning
(which is fine, as it does help to keep track of the Staff lines when
changing the spacing)

whereas

\new Staff \with {
  \override StaffSymbol #' staff-space  = #1.2
} {a b c' d'}

does not? (It produces a standard Staff, without any vertical
delimiter on the left).

What is the exact value that triggers the behavior? Is it documented
anywhere (couldn't find it)?

If it's intended and this threshold is definable, it's definitely a
feature more than a bug.

Cheers,
Valentin