Incorrect stem-lengths of beamed notes when Staff Y-extent is set

[Daniel Johnson]

%%% BEGIN LILYPOND CODE %%%
\version "2.8.0"

\paper {
   indent = 0\in
   raggedright = ##t
}

theNotes = \relative c' {
   f8[ g f e] c[ d] aes8[ b] | g'8([ aeh g fis]) c8([ d c b])
}

\score {
   \new Voice = theVoice { \theNotes }
   \layout {
       \context {
           \Staff
           \override VerticalAxisGroup #'minimum-Y-extent = ##f
           \override VerticalAxisGroup #'Y-extent = #'(-3.5 . 4)
       }
   }
}
%%% END LILYPOND CODE %%%

This results in incorrect beamed stem-lengths as seen the attached 
image.  If you comment out the Y-extent override, the problem goes 
away.  If you comment out only the minimum-Y-extent, the output changes 
slightly but the problem remains.  This has been confirmed in 2.8.1 and 
2.9.2.

I use the Y-extent hardcode to establish staff distances for SATB music 
with lyrics.  I suppose I could look at the PianoStaff code and see how 
it establishes a fixed distance between the staves in a system, but that 
will mean lots of manual editing. My other alternative is to revert to 
one of the later 2.7 versions.

--Daniel