Access the first/last line of a markup

[Urs Liska]

Hi,

returning to my notorious "center-markup-over-measure" issue I was 
pointed to yet another issue: vertical alignment when the last line 
contains descenders or the first line does not contain ascenders.

Since I am manually placing the markups (given as arguments) using 
ly:stencil-combine-at-edge above and below the staff I can't use 
staff-padding for padding against the baseline. So what I *think* I need 
is retrieving the first and last lines of the markup (which would be the 
same if it's a single line markup) and analyze that. However, that seems 
difficult. I checked with \displayMusic -\markup ... to see if I could 
retrieve stuff from that expression, and although it seems 
complicated-but-feasible for most cases it would fail with e.g. 
\wordwrap because there's no notion of the resutling lines yet.

I can also access the resulting stencils after using 
grob-interpret-markup. However, I have no idea if/how it is possible to 
get to the information I need (either a "sub-stencil" for the first/last 
line or some information through the stencil-extent).

Do you think there's any automatic solution I can pursue - or will I be 
left manually adjusting the padding on a case-by-case basis?

Thanks for any suggestions
Urs