I often write in French and, according to typography rules, I use narrow no-break spaces (U+202F) between a word and a "double" punctuation mark that immediately follows it (said marks: ?, !, ;, ;, «, »).

When I do that, the spell checker in all KDE apps (so the common denominator should be sonnet?) wrongly considers the word, the NNBSP and the punctuation do be the same word, which of course doesn't exist.

"Bonjour !" (with standard space character) -> spell checks as correct
"Bonjour !" (with NNBSP -> spell checks as incorrect; the whole string is underlined with squiggles. Expected: the spell checker should say this text is correct.

As a side note, the wider non-break space (U+00A0) does break words as it is supposed to.
"Bonjour !" (with NBSP) -> spell checks as correct

I am trying to guess where the issue comes from. It seems sonnet uses QTextBoundaryFinder as a backend. But it isn't clear that this library is meant for spell checking, as opposed to deciding where to break the text at the end of a line in a text area (non breakable spaces should prevent breaking lines, but still separate words for spell checking intents). Maybe this is not the right track though, since NBSP and NNBSP behave differently.

Another remark: using aspell directly in the command line breaks the words correctly.