Can't annotate Non-rhythmic items for single staff only

[uliska]

When non-rhythmic elements (usually living in the staff context?) are highlighted with a span the outcome will be visible in the whole score:

\include "oll-core/package.ily"
\loadModule scholarly.annotate

one = {
  c'1 c'1
}

two = {
  \clef bass
  c1
  \lilypondIssue "It is not possible to mark just *this* staff's barline"
  \bar "|"
  c1
}

\score {
  <<
    \new Staff \one
    \new Staff \two
  >>
}

This seems to be related to the fact that these items are determined as 'non-rhythmic here:

https://github.com/openlilylib/stylesheets/blob/master/span/module.ily#L482-L488

and then assigned the music-type 'once which makes them applied through overrideProperty here:

https://github.com/openlilylib/stylesheets/blob/master/span/module.ily#L206-L211

For some reason overrideProperty behaves that way, as opposed to \once \override:

one = {
  c'1
  \once \overrideProperty Staff.BarLine.color #red
  \bar "|"
  c'
  \once \override Staff.BarLine.color = #magenta
  c'
}

two = {
  s1*3
}


\score {
  <<
    \new Staff \one
    \new Staff \two
  >>
}

[uliska]

\overrideProperty uses an engraver living in the Score context, therefore it applies to that context too.
See http://lilypond.org/doc/v2.19/Documentation/notation/available-music-functions#index-overrideProperty-1
and https://lists.gnu.org/archive/html/lilypond-user/2019-02/msg00374.html

In theory it works to move Output_property_engraver to a lower context but that sounds pretty fragile:

• It would have to be added to all lower-level contexts
• sometimes it does make sense to affect the Score context
• It wouldn't be clear whether it should be the Staff or the Voice level.

\layout {
  \context {
    \Score
    \remove Output_property_engraver
  }
  \context {
    \Staff
    \consists Output_property_engraver
  }
}

does work with the MWE, but it seems not the way to go forward.

Instead I'll have to see if I can avoid using (overrideProperty), maybe by reviewing the whole design of the engraver (what is done at what point in time?).