avoid-slur strange behavior with TextScript

[Gilles THIBAULT]

Hello.

avoid-slur  with TextScript objects doen't seem to work a usual :

%%%%%%%%%
\version "2.11.42"

\relative c'' {
    \time 2/4
    fis4( f
    e ees
    \override Voice.TextScript #'avoid-slur = #'inside
   d2^\markup \column {\flat \musicglyph #"scripts.trill"  }
    c4)
}
%%%%%%%%

However, it seems that it reserves space for the TextScript object but 
doesn't move it like for other object having this property.
So a work-arround is easy to find by adding :
    \once \override Voice.TextScript #'extra-offset = #'( 0 . -6.8 )

But is it a bug or a special behavior with TextScript #'avoid-slur ?

Gilles