Proportional notation broken by accidentals and short notes

[to7m]

% uniform-stretching causes bad bar line placement in the
% middle system. The accidentals and notes shorter than
% proportionalNotationDuration use uneven spacing to avoid
% collisions, despite strict-note-spacing.

\version "2.18.2"\version "2.18.2"

\layout { ragged-right = ##t }

TheGs = { g8 g g g g g g g }
Flats = { ges8 g ges g ges g ges g }
Semis = { g16 g g g g g g g g g g g g g g g }

\score {
  \new Staff { R1 \break
               \TheGs \TheGs \TheGs \break
               \Flats \Flats \Flats \break
               \Semis \Semis \Semis }
  \layout {
    \context { \Score
      proportionalNotationDuration = #(ly:make-moment 1 8)
      \override SpacingSpanner.uniform-stretching = ##t
      \override SpacingSpanner.strict-note-spacing = ##t
    }
  }
}