A beamlet like with { c''8[ c''16] } is extended to the NonMusicalPaperColumn if broken.
\version "2.19.82"
{
\override Beam.breakable = ##t
s8*7 c''8[ \break c''16]
}
Desired behaviour would be to keep the default beamlet length. Or at least an easy method to change behaviour.
Workaround is available at
http://lists.gnu.org/archive/html/lilypond-user/2018-10/msg00406.html
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Diff: