Performance/visual regression for URL previews

[mistydemeo]

Steps to reproduce

1. Paste a link in a room.
2. Observe that a link preview is displayed.
3. Switch to another room.
4. Switch back.

Outcome

What did you expect?

The URL preview is displayed right away with the rest of the room content.

What happened instead?

The URL preview loads in after a half-second delay. While that's not a big deal, it does mean that the URL previews loading in shifts the position of other content further up/down in the timeline to make room for it. Prior to this, the URL previews already being present on load meant that content positions remained stable.

This is related to #28758 (thank you for the lightning fast fix for that issue!). I'm guessing that, like with that issue, this is a result of the changes made in #28579.

Operating system

macOS 15.1.1

Application version

Element 1.11.89

How did you install the app?

https://element.io/download

Homeserver

Synapse 1.121.1

Will you send logs?

No