While simple cases for leaders with leader-pattern="use-content" work
there are a few more difficult to attack gremlins left.
When it comes to Knuth line breaking leaders are modelled as non
breaking elastic glue. This allows for leaders under justification to
fill the remaining space and works fine.
However lets take this example:
<fo:leader font-size="24pt" leader-pattern="use-content">x</fo:leader>
The rest of the line
Because the leader just becomes a piece of Knuth glue the Line LM
doesn't know that it is actually a "24pt" font and doesn't calculate
the line height correctly. Of course this is 'getting worse' when one
thinks that the leader could contain nested inlines of different
sizes/alignment/baselines/... All this information which is in some
form or another normally used by the Line LM to calculate the line
height and by the inline LMs to perform vertical alignments is not
Don't think this is urgent and there is a disabled test case now for it.
Are there other cases where we could have similar problems, i.e. the
Knuth sequences not carrying all the information expected because some
substitution is done - page number citations, markers, ....?