After reloading, E2E rooms can be stale/blank with no spinner for ages

[ara4n]

I just refreshed develop post the 0.17 release, and got a stale view of my E2E rooms (showing the last time the sync accumulator was persisted, i guess). However, it then takes minutes for the missing messages to appear, and during that time, there is no spinne visible. Sometimes the timeline shown can be entirely blank

[turt2live]

Duplicate of #7116 (which is claimed as cannot reproduce?)