Previews when composing a message

[chrisbobbe]

In the web app, we let the user double-check how their message will be rendered before they send it:

We should do the same in the mobile apps. (I didn't find an issue for this in zulip-mobile in a quick search.)

Wanting this feature probably isn't quite as common in mobile, because one tends to stick to shorter messages with simpler formatting. But that's far from absolute.

Help Center doc: https://zulip.com/help/preview-your-message-before-sending

Implementation

The implementation should use this server endpoint to turn the draft text into HTML:

• https://zulip.com/api/render-message

and then parse the HTML the same way we do for an actual message.

A prerequisite issue is:

• content: Support Zulip content outside messages (even outside per-account contexts) #488

[gnprice]

We had a user request for this feature last month:
https://chat.zulip.org/#narrow/channel/48-mobile/topic/iOS.20feedback.20for.20Julia.20community.20member/near/1961857
https://chat.zulip.org/#narrow/channel/48-mobile/topic/iOS.20feedback.20for.20Julia.20community.20member/near/1963803

They raised code blocks and LaTeX as particularly useful situations for a preview.

[chrisbobbe]

This came up in beta feedback today: https://chat.zulip.org/#narrow/channel/48-mobile/topic/message.20previews/near/2010648

There is also no preview for message prepared to be published.