The UnifiedHighlighter's DefaultPassageFormatter (mostly unchanged from the old PostingsHighlighter) will format overlapping matches by closing a tag and immediately opening a tag. I think this is a bit ugly structurally and it ought to continue the tag is if the matches were merged. This is extremely rare in practice today since a match is always a word, and thus we'd only see this behavior if multiple words at the same position of different offsets are highlighted. The advent of matches representing phrases will increase the probability of this, and indeed was discovered while working on
LUCENE-8286. Additionally, and related, OffsetsEnums should internally be ordered by the end offset if the start offset is the same.