sss: timeline resets when sync completes, causing a scroll jump

[ara4n]

Steps to reproduce

1. launch app
2. open room and scroll back to read scrollback while it syncs
3. once sync completes and spinner stops, the timeline resets and scrolljumps you unceremoniously to the bottom of the timeline

Outcome

What did you expect?

never scrolljump, never visibly reset the timeline

Your phone model

No response

Operating system version

No response

Application version

683

Homeserver

No response

Will you send logs?

Yes

[ara4n]

in fact, it's worse than this. most of the time when i open a room on build 686, the timeline resets while i'm reading scrollback. it's like it's resetting once a subscription or /messages completes or something. having the UI disappear and reappear and fling you back to the bottom of the timeline while you're trying to read it is 😬

[ara4n]

this continues to be a really nasty wart with sss (especially if it conspires with #3214 and similar to lose msgs entirely)

[ara4n]

I continuen to get this almost every time i open a room on sss. It makes the app feel distinctly flakey.