In highlight_mark(), mark_text was sliced using byte-based indexing
(mark_text[:len(hint)] and mark_text[len(hint):]). Since len(hint)
equals the rune count (hint is always ASCII), but len(mark_text) is a
byte count, this could slice in the middle of a multi-byte UTF-8
sequence (e.g. CJK characters), producing an invalid byte sequence
rendered as the Unicode replacement character (�).
Fix by converting mark_text to []rune first, then slicing at rune
boundaries. The hint is ASCII so len(hint) == rune count, requiring
no conversion on the hint side.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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