Re: Issue 3641: Keep only one Axis_group_engraver active (issue 20500043)

[k-ohara5a5a]

https://codereview.appspot.com/20500043/diff/20001/lily/vertical-align-engraver.cc
File lily/vertical-align-engraver.cc (right):

https://codereview.appspot.com/20500043/diff/20001/lily/vertical-align-engraver.cc#newcode99
lily/vertical-align-engraver.cc:99:
The contexts with a Vertical_align_engraver, StaffGroup and such, would
not normally have the Axis_group_engraver that sets hasAxisGroup.

If you add the Axis_group_engraver to a StaffGroup, setting the contents
of that StaffGroup as if all on one Staff, then I see how you might not
want Vertical_align_engraver deciding whether to draw braces and
creating StaffGroupers with spacing parameters that do not make sense.

This removal of the Vertical_align_engraver looks like a convenience
feature, just for cases where you add Axis_group_engraver to an unusual
context.

Usually, though, when users move an engraver from one context to
another, we add and remove individual engravers explicitly.
 \new StaffGroup \with {\consists Axis_group_engraver } <<
   \new Staff \with {\remove Axis_group_engraver }
   \new Staff \with {\remove Axis_group_engraver } >>
so it is good to restrict automatic removals to cases that are certain
errors, and to print the warning.

https://codereview.appspot.com/20500043/