OK, a little experimentation showed that the right syllable, -bar, is not shifted on the first line because, for some reason, there is no stencil for the non-printed hyphen, yet there is one for the non-printed hyphens on subsequent lines. This means that the variable sten-ex gets assigned the empty interval, and no shift occurs.
I don't know the best way to solve this, but this should at least work.
This definitely improved the situation… but it still leaves gaps under certain circumstances, e.g. here’s the fourth line (starting at m28):
I think [to begin with] there are two separate issues to consider here:
1. In m.28, it’s clear that “foo bar” has not become one Pango string, which it apparently has on the downbeat of m.29 (and really “should”, I imagine, in any “ultimate fix”).
2. If “foo-bar" happens over a barline, it probably shouldn’t be compressed; at the very least, that should be a settable property (analogous: #’to-barline).
Thanks,
Kieren.
p.s. I know this isn’t likely to be a super-quick fix — if it becomes too big, let’s negotiate a bounty amount. =)