Element stops asking for history if it's full of reactions

[timokoesters]

Steps to reproduce

Join a room with a lot of reactions after another.

In my testing I used the bug from #25872

Outcome

Sometimes it is also stuck showing a spinner.

Operating system

No response

Browser information

No response

URL for webapp

No response

Application version

No response

Homeserver

No response

Will you send logs?

No

[t3chguy]

Toggling canBackPaginate a couple of times caused more history to load :D

[t3chguy]

Looks like the issue is due to Element dropping the events due to not knowing what timeline (main or thread) they belong to
so then other code assumes the server messed up and didn't send us any events and gave up
this won't be an easy fix unfortunately

matrix-org/matrix-spec-proposals#4023 would make this problem go away

Edit:
this can also happen even if the MSC lands with threads until we have a filter for backpaginating excluding threads - matrix-org/matrix-spec-proposals#3874

So the proper fix requires both MSC4023 and MSC3874

[t3chguy]

The above PRs are workarounds until the MSCs have wide availability