Controlling spanner visibility after a line break
The visibility of spanners which end on the first note following a line break is controlled by the after-line-breaking callback ly:spanner::kill-zero-spanned-time.
For objects such as glissandos and hairpins, the default behaviour is to hide the spanner after a break; disabling the callback will allow the left-broken span to be shown.
Conversely, spanners which are usually visible, such as text spans, can be hidden by enabling the callback.