#5020 Grace notes obscure ledger lines

[Graham Percival]

Minimal working example:

\version "2.18.2"

\markup{"Grace note beams and slash cover ledger lines:"}
{
  \clef bass
  \grace{
    e,,32 f,, e,, f,,
  }
  a,,2
  \slashedGrace{e,,8}
  a,,2
}

More info:

\markup{"Perhaps better would be:"}
{
  \clef bass
  \once \override Beam.positions = #'(-0.5 . -0.5)
  \grace{
    e,,32 f,, e,, f,,
  }
  a,,2
  \once \override Stem.length = #11
  \slashedGrace{e,,8}
  a,,2
}

\markup{"Note that regular notes do not show this problem:"}
{
  \time 5/4
  \clef bass
  e,,32 f,, e,, f,,
  a,,2
  e,,8
  a,,2
}

More discussion here:
http://lists.gnu.org/archive/html/bug-lilypond/2016-12/msg00050.html