Balloon fails with cross staff arpeggio

[Colin Campbell]

Bertrand wrote:
Example :

\version "2.15"
\new PianoStaff \with { \consists "Balloon_engraver" } <<
  \set PianoStaff.connectArpeggios = ##t
 { \balloonGrobText #'Arpeggio #'(-0.1 . 0) \markup "G.O." \repeat unfold 5
a''\arpeggio }
 { \repeat unfold 5 a\arpeggio } >>

Regards,
Bertrand
###########

I've confirmed this as written with 2,12,3, 2.13.61 and 2.15.0.
I've also discovered that setting \balloonLengthOn hels the collision with the
time signature. The bigger change comes from removing Balloon_engraver from the
PianoStaff and putting it into explicitley declared Voices.  This gives a 
partial
balloon in the upper voice and a complete but probably wrongly-sized one in the
lower.

Bertrand's original example generates this message: "warning: vertical 
alignment called before line-breaking.", which appears in issue 341.

Before raising an issue, would the devs have further advice and guidance
for Bertrand and me?

If it is indeed an issue, I would not call it a regression because it fails
in 2.12.3 as well as current devel.