Mentions autocomplete does not work properly with @ as instance separator

[Nothing4You]

Requirements

• This is a bug report, and if not, please post to https://lemmy.ml/c/lemmy_support instead.
• Please check to see if this issue already exists.
• It's a single bug. Do not report multiple bugs in one issue.
• It's a frontend issue, not a backend issue; Otherwise please create an issue on the backend repo instead.

Summary

When only a single @ is used, autocompleting usernames for mentions works and the text is replaced as desired.
When a second @ is added at the end to indicate the end of the username, which separates the username from the instance name, this gets treated as the start of the mention once again, rather than extending the previous mention autocomplete.

Raised from https://sh.itjust.works/comment/13097608

Steps to Reproduce

1. Type @threelon in a comment
2. Autocomplete suggests @[email protected]

3. Click autocomplete option, which inserts [@[email protected]](https://sh.itjust.works/u/threelonmusketeers)
4. Start new mention, type @threelonmusketeers@
5. Autocomplete suggests the same. Sometimes this also moves the autocomplete suggestion to the second @ instead of the first one

7. Clicking the autocomplete option inserts [@[email protected]](https://sh.itjust.works/u/threelonmusketeers) in the place of the second @, leaving @threelonmusketeers[@[email protected]](https://sh.itjust.works/u/threelonmusketeers) as the full text
8. Start a new mention, type @threelonmusketeers@sh
9. Autocomplete only searches for the text after the second @

Technical Details

Firefox 128.0.3

Lemmy Instance Version

0.19.5

Lemmy Instance URL

https://voyager.lemmy.ml

[dessalines]

I don't have time to work on this one, but the autocomplete was never supposed to work with multiples of the same symbols (and I have no idea how that would work).