#5548 staffgroup-staff-spacing may be applied multiple times per StaffGroup

[Simon Albrecht]

Reported by Harm:

for nested container-contexts StaffGrouper.staffgroup-staff-spacing is
applied not only to the last Staff (as opposed to the NR).
See code below, png attached.

\version "2.19.82"

mus = \new Voice { c''1 }

\score {
  <<
    \new StaffGroup
      <<
        \mus
        \new GrandStaff << \mus \mus >>
        \mus
        \new GrandStaff << \mus \mus >>
        \mus
      >>
    \mus
  >>
  \layout {

      %% Play with the padding-values
    \override StaffGroup.StaffGrouper.staffgroup-staff-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 10)
       (stretchability . 0))
    \override GrandStaff.StaffGrouper.staffgroup-staff-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 0)
       (stretchability . 0))

      %% Don't get disturbed by other settings
      \override Staff.VerticalAxisGroup.default-staff-staff-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 0)
       (stretchability . 0))
    \override StaffGroup.StaffGrouper.staff-staff-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 0)
       (stretchability . 0))
  }
}

http://lilypond.org/doc/v2.19/Documentation/notation/flexible-vertical-spacing-within-systems
states:

"Properties of the StaffGrouper grob
[...]
staffgroup-staff-spacing
The distance between the last staff of the current staff-group and the
staff just below it in the same system,
[...]."

Actually, the last Staff of a StaffGroup (or equivalent) and every
Staff right before another container-context is spaced by
staffgroup-staff-spacing.

I'm undecided whether I'd call this behaviour a bug or a feature.