untangling the remaining regressions

[Keith OHara]

When solving regressions we have the choice to go forward or to retreat --
to repair, or revert, the change that caused the regression.

(1) The positioning of fingerings using the outline of the note-column (the 
last patch under issue 2527) is a user-visible improvement, so it would be nice 
to keep it.  I have posted patches to solve two issued (3465 and 3363) were 
caused by the fingering patch, so maybe we can go forward.

The fingering patch might also have made issue 601 worse, and possibly 
re-broken issue 40.  I hope one of use can bisect to isolate the cause of these.


(2) The expanded use of unpure-pure containers (issue 3199) is a reorganization 
of the code.  Overall, I think the patch makes things simpler, conceptually.  I 
think the remaining problems that it caused (issue 3385, issue 3359 and 
blocking the patch for issue 3363) come from its removal of the test 
'pure-relevant?', which ignored some objects for purposes of estimating the 
heights of staves for the planning of line/page-breaking (i.e., the 
pure-heights of VerticalAxisGroups).   When we have objects that cross staves, 
and we include them in the pure-height of one of the staves, that height 
depends on page-layout, which we have not yet determined.

So we can revert 3199 with no loss except going back to the old more-complex 
code, or try to repair, probably by reimplementing a filter to do some of what 
'pure-relevant?' used to do.  (My thinking now is that for a Grob to be 
pure-relevant to a Staff, it must have that Staff as a (grand*)-parent.)

Does anyone see reason to try to move forward, to repair the 
unpure-pure-containers patch (3199) ?