The various classes of Line objects in a Sibelius score appear to have two independent properties affecting magnetic layout:
(1) whether the Line avoids other objects in the score by displacing itself;
(2) whether the Line pushes staves apart during Layout -> Optimize.
Just a few classes of Line have both properties (for instance: hairpins, trills, pedals). Most have only (1) and not (2). Some have neither.
Nowhere in the user interface does there appear to be any means by which the user can enable/disable these properties of Lines.
So, for instance, creating a "poco cresc." dotted line which can be placed between the two staves of a Grand Staff can be quite a challenge, as dotted lines do not have the property of pushing staves apart to make room for the Line.
At present, a work-around is to copy a Trill Line and re-design it to say "poco cresc." with a dotted line. But such a re-designed line still, by default, has the playback properties of a trill, and does not have the same playback properties as a hairpin.
How much easier things would be if one could simply edit the properties of a dotted line to have the "Push staves apart" property. So this Idea is to propose that Lines should have these properties exposed somewhere in the user interface.
I have attached a test Sibelius 7.5 score demonstrating which classes of Line have which magnetic properties.
The original discussion on the Sibelius forum is at: