Require to wait for letter following "@" when mentioning another user before showing options: parity across platforms

[daniellekirkwood]

The decision has been made to only show the list of people in a room when using the @ mention capability when the user has type a letter follow "@"

The discussion linked shares more info.

Andoird is currently the only platform that does not act this way so this issue is to address that and bring all platforms to the same.

If we choose not to do this in the legacy apps then feel free to close this and re-open in an EX workspace.

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Discussed in element-hq/element-meta#1032

^{Originally posted by daniellekirkwood February 22, 2023}
When the user types "@" on mobile a list of options shows on Android.

On iOS and Web, the user must type "@" followed by a character.

We should align on which we'd like to implement and the create the issues on the corresponding platforms to make that the case.

Reference: element-hq/element-web#24018

In web, this looks like: