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