possible fix for 5026 "\crossStaff mangles all grace note flags"?

[Thomas Morley]

Hi,

I had a look at
issue 5026 "\crossStaff mangles all grace note flags "
https://sourceforge.net/p/testlilyissues/issues/5026/

The code below checks additional for equal stem-durations.
If true connect stems, if not don't do so _and_ unset Flag.style.

Though, I'm not sure it's a proper fix.

I'd appreciate opinions/testings.


Thanks,
  Harm

\version "2.19.52"

#(define-public (my-Span_stem_engraver ctx)
  "Connect cross-staff stems to the stems above in the system"
  (let ((stems '()))
    (make-engraver
     ;; Record all stems for the given moment
     (acknowledgers
      ((stem-interface trans grob source)
       (set! stems (cons grob stems))))
     ;; Process stems and reset the stem list to empty
     ((process-acknowledged trans)
      (let ((flags
              (flatten-list
                (map
                   (lambda (stem) (ly:grob-object stem 'flag))
                   stems)))
            (stem-durs
              (map
                (lambda (stem) (ly:grob-property stem 'duration-log))
                stems)))
      ;; condition to connect:
      ;; connect stems only if two or more present _and_ all are of same
      ;; duration and `stem-connectable?' is true (checked earlier)
      (if (and (<= 2 (length stems))
               (every (lambda (el) (= (car stem-durs) el)) stem-durs))
          ;; `make-stem-spans!' is not public, thus the @@-syntax
          ;; can be deleted if inserted in music-functions.scm
          ((@@ (lily) make-stem-spans!) ctx stems trans)
          ;; if the condition is false unset Flag.style
          (if (pair? flags)
              (ly:grob-set-property! (car flags) 'style '())))
      (set! stems '()))))))

A = {
  c'8 \grace d'8 e'8 f' \grace g' fis' g'4 a'8 \noBeam b' c''8
}

B = {
  \voiceOne
  \autoBeamOff
  \crossStaff { c'8  \grace {  d'8 } e'8 \grace e'8 f' fis' g'8 a' b'4 }
  c''8
}

music = <<
  \new Staff
    %% works only if patch for issue 4983 is present
    %\with { \override Flag.stencil = #modern-straight-flag }
    \A
  \new Staff
    %% works only if patch for issue 4983 is present
    %\with { \override Flag.stencil = #flat-flag }
    \B
>>

\new PianoStaff
  \with {
    \consists #Span_stem_engraver
    instrumentName = "current"
  }
  \music

\new PianoStaff
  \with {
    \consists #my-Span_stem_engraver
    instrumentName = "proposed"
  }
  \music