Re: `\offsetPosition` fails

[Werner LEMBERG]

> This is because it assumes that Slur.control-points is still set to
> its original callback at the time `after-line-breaking` is run,
> which isn't true if something like
> `ly:side-position-interface::move-to-extremal-staff`, in
> `BarNumber.after-line-breaking`, reads `control-points` earlier,
> making `(ly:grob-property-data grob 'control-points)` return the
> computed value.  A good example of how `after-line-breaking` can be
> fragile.
> 
> Try [...]

This works, thanks!

> I can see why the original snippet uses `after-line-breaking`: using
> `ly:slur::calc-control-points` directly here is suboptimal (needs
> updating if the default changes).  The use of
> `ly:grob-set-property!` isn't wonderful either.  Unfortunately, the
> slur code would need to be structured a bit differently to get rid
> of the latter one, although you could, if you care enough and with a
> bit more effort, address the former by creating a variant of
> `grob-transformer` passing the raw original callback to the
> transformer instead of the value it computes.

Well, the slur code is something that we have to tackle eventually,
but I guess it will take some time until this happens.

I have already updated https://lsr.di.unimi.it/LSR/Item?id=748 with
your code and will add this snippet to the NR soon.


    Werner