TLDR: they implemented case folding with a lot more SIMD via autovectorization.
> almost every fold preserves the UTF-8 length or shrinks it, but two outliers grow—U+023A (Ⱥ) and U+023E (Ɀ) are 2 bytes each yet fold to 3-byte characters (ⱥ, ɀ)
Fix this by reversing it. Fold ⱥ to Ⱥ instead of the other way around. The search index won't only consist of lowercase characters any more, but that never mattered.
> almost every fold preserves the UTF-8 length or shrinks it, but two outliers grow—U+023A (Ⱥ) and U+023E (Ɀ) are 2 bytes each yet fold to 3-byte characters (ⱥ, ɀ)
Fix this by reversing it. Fold ⱥ to Ⱥ instead of the other way around. The search index won't only consist of lowercase characters any more, but that never mattered.