markLengthOn leads to excessively wide MMRs

[Saul Tobin]

\layout {
  \context {
    \Score
    skipBars = ##t
    \markLengthOn
  }
}

{
  \tempo "Tempo" 4=120
  R1*4
  \mark\default
}

The MMR should only stretch to the minimum length necessary to
accommodate the MetronomeMark. This leads to very ugly results with long
MetronomeMarks, including sometimes stretching measures beyond the page
margins (even if the text itself fits within the margins). It also leads to
ugly results when a system contains multiple MMRs, as one is too wide and
the other too narrow.

I have not found an obvious way to tweak this behavior.