Odd interaction between ly:grob-property and hairpin

[Simon Albrecht]

Hello list,

if a grob callback queries a grob property on a hairpin as follows, the 
hairpin gets printed infinitely short (with a corresponding warning: 
crescendo too small):

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% tested with version 2.22.0 as well -- same bug
\version "2.23.6"
% guile2 binary from
% 
https://gitlab.com/lilypond/lilypond/-/releases/release%252F2.23.6-1/downloads/lilypond-2.23.6-linux-x86_64.tar.gz
off =
#(define-event-function
  (ev)
  (ly:music?)
  (let ((offsetter-fn
         (lambda (grob)
           (let* (
                   ; comment/uncomment the following to trigger bug:
                   ; if the grob property is queried, the hairpin gets 
infinitely short
                   (yoff-prev (ly:grob-property grob 'Y-offset 0))
                   )
             (ly:grob-set-property! grob 'color red)
             ))))
    #{
      - \tweak before-line-breaking $offsetter-fn
      - $ev
    #}))

{ 2\off \< 2\f }
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Best, Simon