Strange behavior at the beginning of a StaffGroup, GrandStaff and PianoStaff

[Clément Ryder]

Hello,

Since version 2.19 (I don't know which minor version), the beginning of a
StaffGroup / GrandStaff / PianoStaff behaves oddly.
It is obvious in the “Horizontally aligning custom dynamics” snippet :
v2.19 : http://lilypond.org/doc/v2.19/Documentation/41/lily-e77ad0e2.png
v2.18 (ok) :
http://lilypond.org/doc/v2.18/Documentation/22/lily-8c35732c.png
There is a kind of *invisible wall* at the beginning of the first system,
which dynamics cannot break.
All staves are affected. The bug sill occurs in a single-Staff Staffgroup.

ChoirStaff works great though.
Playing with X-offset and extra-offset (which ignores the *invisible wall*)
can be a quite correct workaround.

Clément Ryder