Once I learned about outside staff-priority, everything was good.
Which for me is the usual situation: I get unexpected output, and
then learn that there's a perfectly acceptable reason for that
output. In this case: outside-staff-priority takes precedence over
avoid-slur. The learning manual and exra-offset were never needed.
However, there are situations where you don't have access to the
mailing list, and then you have to find it in the docs. Can you
suggest where improvements shall be added so that your specific
problem is covered?