Re: Bug: restarting staff destroys DynamicLineSpanner.staff-padding after line break

[Aaron Hill]

On 2019-03-06 4:55 pm, Trevor Bača wrote:
> Hi,
>
> Restarting the staff during the lifespan of a multisystem
> DynamicLineSpanner destroys the value of DynamicLineSpanner properties
> (like staff-padding) that were set when the DynamicLineSpanner was 
> created.
>
> In the MWE below, the hairpin should exhibit staff-padding equal to 10
> staff spaces below all four systems; but the value of staff-padding is 
> lost
> at the point (in system 2) that the staff is restarted; we see evidence 
> of
> this after the next line break (in systems 3 and 4) where no staff 
> padding
> appears.

staff-padding is not destroyed.  It is still set.

The issue is the broken spanners have lost their reference to the 
StaffSymbol.  And Side_position_interface::aligned_side 
(side-position-interface.cc:180) expects that there is a valid 
StaffSymbol in order to care about staff-padding.

Add the following \override to your code and note the output:

%%%%
  \override DynamicLineSpanner.after-line-breaking = #(lambda (grob)
    (display (format "~a ~a ~a\n"
      (ly:grob-property grob 'staff-padding)
      (ly:grob-property grob 'Y-offset)
      (ly:grob-object grob 'staff-symbol))))
%%%%

> Drawing systems...10 -12.05 #<Grob StaffSymbol >
> 10 -12.05 #<Grob StaffSymbol >
> 10 -4.57775475073964 #<undefined>
> 10 -5.45702919685039 #<undefined>

You can see that the staff-padding is still 10 for all of the spanners.  
But only the first two that have a valid StaffSymbol end up with the 
expected Y-offset.


-- Aaron Hill