RFC and help at MR 2256

[Thomas Morley]

Hi,

https://gitlab.com/lilypond/lilypond/-/merge_requests/2256 attempts to
solve https://gitlab.com/lilypond/lilypond/-/issues/3176 by
introducing AgogicMark, AgogicSpanner and AgogicLineSpanner at
score-level.
It all mimics how DynamicLineSpanner collects and aligns dynamic stuff.
So far all works fine.
Alas, as soon as the relevant engravers are moved to another context,
which is a common use case, I get an error:

programming error: cyclic dependency: calculation-in-progress
encountered for AgogicLineSpanner.vertical-skylines

and the graphical output is wrong.
Again, this does not happen at score-level.
Example-code to trigger it at #2256

I've no clue how to proceed. Every hint is very welcome.

Thanks,
  Harm