Weird spacing related to barlines

[Phil Holmes]

> I'm not top posting.

Using the gmane interface again, to ensure this gets through.

I'm currently typesetting quite a lot of music from the late 1500's, using 
mensural notation.  With this, it's normal to use

\remove "Default_bar_line_engraver"

to get the closest spacing possible.  However, the down-side is that lines 
don't break.  I discovered that this can be overcome with a simple \bar"" 
early in the music.  Today, I tried to find out how this affected the 
spacing.  It seems quite odd.

With the music below, we see the first block of notes tightly spaced.  
However, without the \bar these run off the side of the page.  Using 
\bar "" to break them, we see the next set of notes grouped into sections 
of 8 notes: it appears that where the end of a measure is mid note, extra 
space is added.  Don't understand.  If I change the pattern of notes so 
that measures always end between notes, the effect disappears.  If I then 
use a wider bar glyph, all the notes are spaced more widely and we still 
see slight evidence of the eight-note grouping.

Can anyone explain what's going on here?

\score {
  \new MensuralStaff {
    \repeat unfold 30 { c''4 c''2 c''4 } c''1 ^"\bar" \bar"" 
    \repeat unfold 40 { c''4 c''2 } 
    c''1 ^"Pattern" \repeat unfold 40 { c''4 c''2 c''4 } c''1 ^"\bar || " 
\bar"||" 
    \repeat unfold 40 { c''4 c''2 } 
    c''1 ^"Pattern" \repeat unfold 40 { c''4 c''2 c''4 } 
  }
  \layout {
    ragged-right-last = ##t
    indent = 0.0
          \context {
            \Score
      \override SpacingSpanner #'spacing-increment = #0
            \remove "Default_bar_line_engraver"
          }
  }
}