"unicode codepoint sequences whose codepoint lengths and/or utf8-code-unit-lengths behave oddly when you change their case" would not fit in a HN title, however
I (OP) said above that "Unicode codepoints that expand or contract when case is changed in UTF-8" would have worked fine, I've changed the Gist title to that in any case. I'm curious if it would've affected the attention it received on HN.