Prefetch sibling messages of a message thread

[ChristophWurst]

Is your feature request related to a problem? Please describe.

When I click a message in my inbox, the full thread loads. The latest message will expand right away, the others stay closed. If I want to expand any of the other messages I have to wait for them to load. Right now this experience isn't fast.

Describe the solution you'd like

Pre-fetch those sibling messages of a thread so that they are already there when a user requests them.

Describe alternatives you've considered

N/a

Additional context

POC done in https://github.com/nextcloud/mail/pull/7059 with the findings that this will bring improvements.