New messages in incomplete threads are invisible

[andybalaam]

Steps to reproduce

1. Have a very old thread, whose thread root is not loaded in your Element Web
2. Get someone to send a new message in that thread

Outcome

What did you expect?

I expect to see the new message somewhere, as per the design comments on element-hq/element-meta#2170 indicating that we should behave as per element-hq/element-meta#1925 .

So a thread should appear in the Thread View, with an error shown at the thread root, but the new message visible inside the thread when I click through.

What happened instead?

The new message was not visible.

[t3chguy]

Does the server return the thread via the thread list APIs in the case you cannot access the root? If not then the thread won't appear until you scroll back to a message containing that thread which is paramount to data loss and likely the real issue here.