Attached is a bit of LilyPond which has a slur over a line break. At the
same moment the clef is changing in the lower staff. The result is a
horribly broken slur start in the top staff.
I blindly applied the dragging-the-shape on this curve which generated
the commented out offsets. This results in something half-way
acceptable. The bug is sensitive to all sorts of things I think - this
was the most I managed to pare it down - reducing the font size made it
go away for example.
Questions:
Is there special syntax for altering the shapes of the half-slurs at
the end and beginning of the line?
Yes, look for \alterBroken in the docs.