Username completion ordering: shorter/exact match sorted after longer incomplete match

# Description

Username completion ordering: when I type a username, `@...` I expect to see the shorter/exact match sorted before the longer incomplete match, but this is not the current behavior.

### Steps to reproduce

- Find/create users with matching-prefix names, ideally where one username is a prefix of the other, `@prefix` & `@prefixuser`
- Type `@` to begin tagging, at `@prefix`
- (I observe) `@prefixuser` first in the suggestions and tab completion, despite that it should come later in lexicographic order.

Is there some kind of special casing of completion on shorter or complete/terminated words/tokens?

Logs being sent: no

### Screenshot
Shows chat text-entry box, with user-completion initiated.
Text: `@dk`
Completion: `@dkess` then `@dk`


### Version information

- **Platform**: desktop

For the desktop app:
- **OS**: macOS
- **URL**: private, 1.7.10