Strange vertical spacing now (2.9.28) with breakbefore = ##t

[Arvid Grøtting]

The spacing is much better now, but it still gets strange in some
not-too-contrived cases:

\version "2.9.19"

#(set-global-staff-size 18)

\header {
  title = "breakbefore spacing test"
  subtitle = "breakbefore = ##t still causes problems"
  piece = "Test"

  texidoc = "Score blocks seem to be badly influenced by neighboring
score blocks (or book titles) that have little or no vertical extent.
This problem only shows up with breakbefore = ##t and multiple staffs.

Visually, it looks like the vertical spacing is influenced by
surrounding pages even when those pages belong to a different piece
and there is a forced page break between pieces.

Ideally, this \book should contain a book title, a number of
identical-looking scores and finally the last, very small score.
"
}

\paper {
  %% annotate-spacing = ##t
  ragged-last-bottom = ##f
}

somemusic = \relative c'' { c4\f d\< e f | g8( a\> g fis g2)\! | 
                            g8.\p( g16) g8( g16\< g g g g g g4) | g1\mf | R1}


thescore = {
  \context Score <<
    \new ChoirStaff <<
      \new Staff <<
        \new Voice = foo { \repeat unfold 16 \somemusic \bar "|." }      >>
      \new Staff <<
        \new Voice = foo { \repeat unfold 16 \somemusic \bar "|." }      >>    
>>  >>
}

\book{
  \score {
    \thescore
    \header {
      breakbefore = ##t
    }
  } 
  \score {
    \thescore
    \header {
      breakbefore = ##t
    }
  } 
  \score {
    \thescore
    \header {
      breakbefore = ##t
    }
  } 
  \score {
    \thescore
    \header {
      breakbefore = ##t
    }
  } 
  \score {
    <<a'1 \\ g'2 >>
    \header {
      breakbefore = ##t
    }
    \layout { 
      ragged-right= ##t
      ragged-last-bottom = ##t
    }
  }

}