Kompare can (apparently depending on font?) convert multiple characters into ligatures, which is not desireable when comparing text. I've seen this with the font Noto Sans Mono. It can quickly be demonstrated with the following (probably requires zsh/bash or similar shell): diff <(echo) <(printf "fi ffi\n1234")|kompare -
Using QFont::PreferNoShaping could help, but it will also disable shaping text in Arabic and Indic text, where this is considered a bug.
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After testing, I can't reproduce this with any font that I've tried. Looks like it's been fixed. Thanks!