Lyric extender lines and rests after line break

[Davide Liessi]

Hi all!

In the following snippet you will notice that the lyric extender after
the line break doesn't start at the beginning of bar 2 but is just a
single dash under the hidden c'4.
The same happens using rests instead of spacer rests.
The glissando is not part of the problem, it is there just to show why
I use spacer rests and hide the final note.
I inserted the line break because the real glissando actually spans
four bars, and for optimal page layout in the orchestral score the
line needs to be broken.

\version "2.16.0"
<<
        \new Voice = "voice" {
                \override Slur #'transparent = ##t
                \override Glissando #'breakable = ##t
                
                c'''4( \glissando
                s4 s2\break
                s2 s4
                \hideNotes
                c'4)
                \unHideNotes
        }
        \new Lyrics \lyricsto "voice" \lyricmode { a __ }
>>

I needed the extender line to start from the beginning of the first
bar after the line break.
I noticed that the problem could be solved by placing a hidden note at
the beginning of each bar spanned by the glissando and skipping that
note using \once \override NoteColumn #'glissando-skip = ##t as in the
next snippet.
This way, wherever a line break could occur, there will be a (hidden)
note at the beginning of the new line.

\version "2.16.0"
<<
        \new Voice = "voice" {
                \override Slur #'transparent = ##t
                \override Glissando #'breakable = ##t
                
                c'''4( \glissando
                s4 s2\break
                \hideNotes
                \once \override NoteColumn #'glissando-skip = ##t
                c'2 s4
                c'4)
                \unHideNotes
        }
        \new Lyrics \lyricsto "voice" \lyricmode { a __ }
>>

Then I tried to tweak the properties of LyricExtender, its interfaces
and Extender_engraver to see if there were ways to do this
automatically, but I couldn't find any.

Does anyone know how to do this without manually inserting hidden
notes in each bar?

Also, is it possible to make the extender line and the glissando line
start and end at the same horizontal coordinates after the line break?
Since I hide the middle and final notes, it would be nice to align the
two lines.

Thank you in advance.
Best wishes,
Davide