BUG OR FEATURE(?): \markup { "Piano " } vs. \markup { "Piano" " " " " }

[Heikki Johannes Junes]

I tried to make a new context with ChordNames in between Piano staves, and found
a strange (?) behaviour with ending spaces in the instrument names:

\version "2.7.5"
melody = \relative c' { \clef "G" c2 b c1 \bar "|." }
harmonies = \chordmode { c'2 g c4 }
accompaniment = \relative c { \clef "F" c2 g c1 }
\score {
  \context PianoStaff <<
    \context Voice = one { \melody }
    \context ChordNames = two { \harmonies }
    \context Voice = three { \accompaniment }
  >>
  \layout {
    \context{
      \PianoStaff
      \override VerticalAlignment #'forced-distance = #7
      % does not work: instrument = \markup { "Piano  " }
      instrument = \markup { "Piano" " " " " " " }
      \accepts "ChordNames"
    }
  }
}

In this example, \markup { "Piano  " } was reduced to \markup { "Piano" }, and
only \markup { "Piano" " " " " } gave the needed extra space. Is this a bug of a
feature the ending spaces are removed?

This behaviour is a bit unobvious, therefore the bug mentioned in
http://lilypond.org/doc/v2.7/Documentation/user/out-www/lilypond/Instrument-names.html#Instrument-names
did not give an adequate advice for adding the extra space needed in the
presence of staff slurs.

Heikki