Anchored exactly on the paragraph top (posOffset 0), LibreOffice's wrap
layout derails and paints a neighbouring inline text box's content
outside its shape; LO itself never writes less than a twip there.
Probed by bisecting a broken box against LO's own DOCX export of the
same document. The importer rounds the sub-visible offset back to none,
so nothing accretes on the round trip.