We have a regression on backfill not being triggered when scrolling to the top on newly joined/created rooms

[matrixbot]

Created by @ matthew:matrix.org.

If you create or join a room, it seems that you can get into the state that scrolling to the top doesn't backfill. This is on current vector.im/beta, so predates vdH's scrolling changes. Matthew saw this on a 1:1 with Jesus, and eman just reported it in HQ too

[ara4n]

I just fixed a related one (#481), but suspect this is still a problem. need to try to repro.

[ara4n]

i'm going to assume this has been fixed between me & vdh, otherwise one of us would have seen it again. will reopen if we see it subsequently...