Strange partcombine Behaviour

[Will Oram]

I've been away from ly for a few months, so forgive me if this has  
been brought up. It doesn't appear in list archives.

Output generated using \partcombine in ly 11.26 looks VERY strange.  
Generally individual lines ignore each other, producing object  
clashes everywhere. An example:

http://www.foxchange.com/spamguy/partcombine.jpg

Solo auto-marking weirdnesses aside (those have been around forever),  
note lines 1, 2, and 6. Glitches include slur clashes, wrong stem  
direction, and overlap of noteheads. Every new version I run the code  
through to update output; because I put lilypond work on hiatus for  
awhile, I only noticed the ugliness now. I have in front of me, too,  
output from March, v11.21. Indeed, the same glitches are visible. I  
definitely don't remember this problem when I was actively observing  
output.

I tried a one instrument partcombine and couldn't reproduce it, so  
the problem lies deeper. I code typical two-part instruments as

\new Staff <<
                        \set Staff.instrumentName = \markup { \hcenter-in #10 "Oboe 
I.II." }
                        \set Staff.shortInstrumentName = \markup { \hcenter-in 
#4 Ob. }
                        \set Staff.midiInstrument = #"oboe"
                        \partcombine
                                \relative c'' { \mIOboeI }
                                \relative c'' { \mIOboeII }
                                \mIOboeDynamics
                >>

...the dynamics part can be commented out or moved, it doesn't  
matter; the output doesn't change. Not sure what else to check, but  
I'll work on producing a sample.

I don't dare succeed
if change always comes first
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