Captioned images can't contain mentions or rich-text

[frebib]

Your use case

What would you like to do?

Have the full composer functionality when sending an image caption, for parity with sending a regular text message. This includes both the rich-text editor view as well as user/room mentions.

Amusingly, since matrix-org/matrix-rust-sdk@8205da8 and some very minor code edits in EXA, this can already be done when editing a caption; just not when sending it to begin with.

Why would you like to do it?

Mainly for functionality parity, but also because it's a pretty common thing to want to do

How would you like to achieve it?

Add a button to the AttachmentPreviewView to enable the rich-text composer and also implement the mention insert view too. There's a huge amount of crossover with the composer in the regular timeline screen, so ideally much of that logic would be deduplicated and reused.

matrix-org/matrix-rust-sdk#4475 should be all that's required on the SDK side. Most of what is missing is UI functionality in EXA.

Have you considered any alternatives?

Sending two separate messages?

Additional context

No response

Are you willing to provide a PR?

Yes