LibreOffice adds the upper block's space-below to the lower one's
space-above, and drops the space-above where a page break put the block
at a page top — but for a Word document it takes only the larger of the
two (all probed against soffice). CSS margins can only collapse, so
space-before rides --space-before and editor.css renders it as padding
('add') or margin ('max'); the model is read from settings.xml, assumed
for DOCX, and written back on ODF export.
