% 2020-11-11 mouseover shows moments of noteheads/barlines (not span; not startbar) 
% updated to \version "2.24.0"

%schemeIndentOn

#(define (url-when fmt) ; cf LSR865
  (lambda (grob)
    (let* (
      (url-strg (format #f fmt (grob::when grob) (grob::rhythmic-location grob)))
      (stil (ly:grob-property grob 'stencil)))
      (if (ly:stencil? stil)
        (let* ((x-ext (ly:stencil-extent stil X))
          (y-ext (ly:stencil-extent stil Y))
          ;; For < 2.22.0 use:
          ;; (url-expr `(url-link ,url-strg ',x-ext ',y-ext))
          (url-expr `(url-link ,url-strg ,x-ext ,y-ext))
          (new-stil 
            (ly:stencil-add
              (ly:make-stencil url-expr x-ext y-ext) 
              stil)))
          (ly:grob-set-property! grob 'stencil new-stil))))))
%                                                                   
%schemeIndentOff

\pointAndClickOff

\layout {
  \override Staff.BarLine.before-line-breaking = #(url-when "BL: ~a ~a") 
  \override  NoteHead.before-line-breaking = #(url-when "NH: ~a") 
}