create a new character style based on another one, let the fontheight be
relative (the actual value is irrelevant), apply both styles to some text. now
change the fontheight of the original style. as a result the relative style
should also change, which is not the case.

I isolated the cause of this issue to some extent. in "SwFmt::Modify", case
RES_ATTRSET_CHG the new and old attribute-sets are getting compared to each
other. The actual work gets done in the function "Differentiate"
[svtools/itemset], which seems to catch redundant attributes from getting
propagated to the derived style. in this particular case this behavior is wrong.
In fact there are identical attributes, but one depends on the other, so it
should not get filtered out.