Empty timeline after lots of scrolling

I scrolled back in a room quite far (to the beginning of August); ended up with an empty timeline with no spinner. I could scroll up and down. Scrolling back down to find the timeline again caused the timeline to load in.

@michaelkaye also had the same experience. We both rageshook.

Reproduce the issue by scrolling far back in a room, observing the empty timeline and missing spinner, then scrolling back down. Start by locating the room timeline's loading and scrolling entry points; done means the timeline remains usable and loading feedback appears while older events are fetched.

Written by the indexing model from the issue text.