Semantics of X-offset Y-offset

[Richard Shann]

The values for X-offset Y-offset when applied to the trill sign or the
coda sign refer to different places on the glyph. For the trill it
refers to the bottom right and for the coda the center right. (I have
found this by trial-and-error).
This causes a problem for dragging these signs on the final typeset pdf
(which you can do in Denemo) because the user expects the symbol to be
dropped centered on the pointer yet there is no consistent offset that
can be applied to achieve this.

Question: where in the LilyPond source tree is the information about
where the "origin" of the glyph is with respect to the center of the
bounding rectangle of the glyph?

Any help is much appreciated.

Richard Shann