2.7.17: minimumVerticalExtent

[Daniel Johnson]

Yesterday I upgraded from 2.7.14 to 2.7.17, and I immediately noticed 
that when trying to set fixed staff-spacing for a staff group, \set 
Staff.minimumVerticalExtent = ##f no longer seems to do anything.  
Consider the following example:

%%% BEGIN LILYPOND CODE %%%
\version "2.7.17"

text = \lyricmode { Hel -- lo world }

\score {
   \context ChoirStaff <<
       \context Staff = women {
           \set Staff.minimumVerticalExtent = ##f
           \set Staff.verticalExtent = #'(-6 . 2)
           \clef treble
           \context Voice = sop { c4 c c2 }
       }
       \context Lyrics = sopLyrics \lyricsto sop \text
       \context Staff = men {
           \set Staff.minimumVerticalExtent = ##f
           \set Staff.verticalExtent = #'(-2 . 6)
           \clef bass
           \context Voice = bas { c4 c c2 }
       }
    >>
}
%%% END LILYPOND CODE %%%

In 2.7.14, the lyrics would be placed exactly centered between the two 
staves, regardless of collision with notes.  However, in 2.7.17 the 
lyrics are pushed down by the low notes in the upper staff.  Is this a 
bug, or has the behaviour of minimumVerticalExtent changed?

--Daniel