Printing rendering of sidebars

Hi,

I'm playing a little with customization layers (thanks a lot, Bob!)
and i'm encountering a problem i don't know how to solve: i think i'm
looking in the wrong direction...

When printing (i'm using XEP), sidebars doesn't appear as i would
like. In fact, i'm using them exclusively with the following form to
grey sample codes backgrounds and draw a border around them:

<sidebar>
  <programlisting>
     ... sample code
  </programlisting>
</sidebar>

When rendering, i find that there is too much padding before the first
line of the programlisting code : a simple inter-line would fit my
tastes...

I've tried to change various attributes of the sidebar.properties
attribute-set (i was thinking that padding-top should do the trick)
but it doesn't change anything.

Now, i think i better have to forget sidebars and customize
programlisting to make it shade its background and draw a frame but,
for the record, i would like to know where my error is.

Any clue?
--
Jaco