Drawing boxes around grobs

The print-function can be overridden to draw a box around an arbitrary grob.

\version "2.24"

\relative c'' {
  \override TextScript.stencil =
    #(make-stencil-boxer 0.1 0.3 ly:text-interface::print)
  c'4^"foo"

  \override Stem.stencil =
    #(make-stencil-boxer 0.05 0.25 ly:stem::print)
  \override Score.RehearsalMark.stencil =
    #(make-stencil-boxer 0.15 0.3 ly:text-interface::print)
  b8
  
  \revert Stem.stencil
  \revert Flag.stencil
  c4. c4
  \mark \default
  c1
}