Mention completions do not appear if typed at the beginning of a line that isn't the first line

Mention completions do not appear if typed at the beginning of a line that isn't the first line

For example, this doesn't give a popup for my username (Moshi on Codidact)

```
some text
@mo
```

These do

```
@mo
```

```
some text
some more text @mo
```