span-bar problem

[Benkő Pál]

hi list,

I have the following problem:
in ancient music there are no real ledger lines,
but the staff is expanded by lines, spanning more
notes (or rests).  LilyPond can do that with
StaffSymbol #'line-positions, but when I combine it
with a score laid out in mensurstrich format, I can
see that extent of the mensurstriche are miscomputed
(as if the involved staves kept their centre).
see the attached example.

I 'fixed' this by the attached patch, but that
does not solve the whole problem: now the extents
are right, but the absolute position is as if the
uppermost staff kept its centre - which is fine as
long as there's no extra line there.

I believe this is caused by Span_bar::print having
a different common refpoint than the context the
result is used in, but I couldn't track it further.
any help and hint is welcome.

p

spantest.ly
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patch17
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