See T3725 for the original discussion of this feature.
- [X] Blur handler wins over click handler, so clicking results doesn't actually work.
- [X] Sorting doesn't put exact username matches first.
- [X] Blur handler still sometimes wins over click handler, depending on how fast things are.
- [X] Excessively aggressive activation when typing ordered lists using `#` as a list marker.
- [ ] Various punctuation rules could likely be more intuitive.
- [ ] Probably better to pop the autocomplete only after the first valid character to prevent flashes, since this is generally better for experienced users.
- [ ] Autocomplete may position itself offscreen, e.g. in Conpherence.