Drawing boxes around grobs

The stencil property can be overridden to draw a box around arbitrary grobs, either using \override or \tweak.

\version "2.24"

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

  \tweak Stem.stencil
    #(make-stencil-boxer 0.05 0.25 ly:stem::print)
  b8

  c4.^"bar" c4

  \override Score.RehearsalMark.stencil =
    #(make-stencil-boxer 0.15 0.3 ly:text-interface::print)
  \mark \default
  c1
}