Call for help with bar lines

[Marc Hohl]

Hello list,

I am quite frustrated – yesterday I managed to get the shift out of
input/regression/beam-feather-breaking.ly which was one of the
last problematic files for my patch.

Today, I looked at the new results: it seems to me that
*every* other regtest file is affected now, but the one mentioned above
is missing – that's not what the patch is supposed to do.

https://www.yousendit.com/download/TEhYa3ZMTEROQnpWUThUQw

(side note: I did not take a full test cycle, since the last
regtest results showed only major changes in a couple of files, most of them
were deliberate, and beam-feather-breaking.ly was the one with the biggest
optical difference, so I checked just this one and did a make all && 
make doc
afterwards, which went trough without complaining).

I double-checked the dimensions of the empty bar line: it is
(0.0 . 0.0) for the current master *and* my patch, so this cannot be
the culprit.

\version "2.17.4"

#(define (my-bar-line::print grob)
  (let ((stencil (ly:bar-line::print grob)))

       (for-each display (list "\n\nBar line extent:" 
(ly:stencil-extent stencil X) " for bar line "
                               (ly:grob-property grob 'glyph-name)))
       stencil))

music = {
  \override Staff.BarLine #'stencil = #my-bar-line::print
  c4 c c c \bar ""
  c c c c \bar "|."
}

\score {
  \new Staff {
    \new Voice { \music }
  }
}


Can anyone with more knowledge than me give me a hint what's wrong?
IIUC correctly, lilypond draws a bar line at the beginning of each line,
but in most cases, this is an invisible one.
If you look at the results of input/regression/lyrics-spanbar.ly,
the whole stuff is shifted that much to the right that lilypond moves
the rightmost rest to a new line! I can't believe that a bar line with
zero width can be the cause for this...

Any hints are highly appreciated!

Regards,

Marc