\version "2.14.2"

% Values are close enough to ignore the difference
#(define (close-enough? x y)
   (< (abs (- x y)) 0.0001))

% Combine a list of extents
#(define (extent-combine extents)
   (if (pair? (cdr extents))
       (interval-union (car extents) (extent-combine (cdr extents)))
       (car extents)))

% Workaround for Lilypond 2.14 - calculate X-extent without the flag
#(define (stem-x-extent stem)
   (let* ((extent (ly:grob-extent stem stem X))
          (layout (ly:grob-layout stem))
          (thickness (ly:grob-property stem 'thickness))
          (thickness-unit (ly:output-def-lookup layout 'line-thickness)))
     (cons (car extent) (+ (car extent) (* thickness thickness-unit)))))

% Check if the stem is connectable to the stem span
#(define ((stem-connectable? ref anchor) stem)
   (cond
    ((inf? (car (ly:grob-extent stem ref X))) #f)
    ((not (close-enough? (car (ly:grob-extent anchor ref X))
            (car (ly:grob-extent stem ref X)))) #f)
    ((< 0 (* (ly:grob-property anchor 'direction)
            (- (car (ly:grob-extent anchor ref Y))
              (car (ly:grob-extent stem ref Y))))) #f)
    (else #t)))

% Connect stems if we have at least two visible stems and the anchor stem
% is one of them
#(define (stem-span-stencil span)
   (let* ((system (ly:grob-system span))
          (anchor (ly:grob-parent span X))
          (stems (filter (stem-connectable? system anchor)
                         (ly:grob-object span 'stems))))
     (if (and (pair? stems)
              (pair? (cdr stems))
              (memq anchor stems))
         (let* ((yextents (map (lambda (st)
                                 (ly:grob-extent st system Y)) stems))
                (yextent (extent-combine yextents))
                (layout (ly:grob-layout anchor))
                (blot (ly:output-def-lookup layout 'blot-diameter)))
           ; Hide spanned stems, but only if it won't hide flags
           (map (lambda (st)
                  (if (close-enough? (cdr (stem-x-extent st))
                        (cdr (ly:grob-extent st st X)))
                      (set! (ly:grob-property st 'transparent) #t)))
             stems)
           (ly:round-filled-box (stem-x-extent anchor) yextent blot))
         #f)))

% Create a stem span as a child of the cross-staff stem (the anchor)
#(define ((make-stem-span! stems trans) anchor)
   (let* ((span (ly:engraver-make-grob trans 'Stem '())))
     (ly:grob-set-parent! span X anchor)
     (set! (ly:grob-object span 'stems) stems)
     (set! (ly:grob-property span 'X-offset) 0)
     (set! (ly:grob-property span 'stencil) stem-span-stencil)))

% Create stem spans for cross-staff stems
#(define (make-stem-spans! ctx stems trans)
   (if (and (pair? stems)
            (pair? (cdr stems)))
       (let* ((anchors (filter (lambda (st)
                                 (ly:grob-property st 'cross-staff)) stems)))
         (map (make-stem-span! stems trans) anchors))))

% Connect cross-staff stems to the stems above in the system
#(define (Span_stem_engraver ctx)
   (let ((stems '()))
     `((acknowledgers
        (stem-interface
         . ,(lambda (trans grob source)
              (set! stems (cons grob stems)))))
       (process-acknowledged
        . ,(lambda (trans)
             (make-stem-spans! ctx stems trans)
             (set! stems '()))))))

\layout {
  \context {
    \StaffGroup
    \consists #Span_stem_engraver
  }
}

\score {
  \new StaffGroup <<
    \new Staff = "staffone" <<
      \new Voice {
        \voiceOne c''4
      }
      \new Voice {
        \override Stem #'cross-staff = ##t
        \voiceTwo c'4
      }
      \new Voice {
        \voiceThree g'4 \stemDown c'4
      }
    >>
    \new Staff = "stafftwo" <<
      \new Voice {
        \clef bass \voiceOne g4
      }
      \new Voice {
        \clef bass \voiceTwo e,4
      }
      \new Voice {
        \clef bass \voiceThree d4
        \override Stem #'cross-staff = ##t
        \stemDown f8 f8
      }
    >>
    \new Staff = "staffthree" <<
      \new Voice {
        \clef bass \voiceOne f4
      }
      \new Voice {
        \clef bass \voiceTwo g,4
      }
      \new Voice {
        \once \override Stem #'cross-staff = ##t
        \clef bass \voiceThree c4 \stemDown \autoBeamOff g8 g8
      }
    >>
  >>
  \layout { }
}