Re: Why is NonMusicalPaperColumn.line-break-system-details.extra-offset "upside down"?

[Kieren MacMillan]

Hi Carl,

> extra-offset affects the position of grobs *relative to the staff containing 
> them*.  When the Y position of a grob is increased, it either has no affect 
> on the location of the staff, or it lowers the staff due to required spacing 
> from the staff above.  Since after-line-breaking is the bottom staff, a 
> positive Y extra-offset value should have no affect or lower the bottom 
> staff, to be consistent with positive Y extra-offset for every other grob on 
> the staff, it seems to me.
> 
> extra-offset doesn’t change the staff-staff spacing parameters, but it does 
> change the grob locations used in the staff-staff spacing calculations.
> 
> I think this is all consistent, but I could be wrong.

<https://lilypond.org/doc/v2.22/Documentation/learning/moving-objects> says:

"extra-offset
This property applies to any layout object supporting the grob-interface. It 
takes a pair of numbers which specify the extra displacement in the horizontal 
and vertical directions. Negative numbers move the object to the left or down. 
The units are staff-spaces. The extra displacement is made after the 
typesetting of objects is finished, so an object may be repositioned anywhere 
without affecting anything else."

From that description, I (perhaps incorrectly?) expect two things:

1. Negative extra-offset [Y-values] in 
NonMusicalPaperColumn.line-break-system-details should move the system *down*.

2. Such a tweak [in any direction] shouldn't be considered at all in the 
staff-staff-spacing calculations.

IMO, either the documentation should be updated to reflect the reality, or the 
behavior of NonMusicalPaperColumn.line-break-system-details extra-offset should 
be modified to match the documentation [and the effect of extra-offset in every 
other situation].

Cheers,
Kieren.