"read to here" line does not appear when scrolling up to last read message or disappears very rapidly

### Description

With the update to 1.7.19, the green "read to here" marker line does not appear when scrolling up to the last read message (detected by the loss of the move to last read message button).

### Steps to reproduce

- Wait for adequate message to arrive
- Scroll up until move to last read message button disappears and then a little bit further
- Look around in bewilderment like Vincent Vega in Pulp Fiction

I would expect the green line to show the boundary between read and unread messages. It does appear sometimes, but very fleetingly. I suspect that it is being removed before being made visible in the cases where it doesn't appear.

Screen shots are not possible here, since this is a dynamic bug.

### Version information

- **Platform**: desktop

For the desktop app:

- **OS**: Ubuntu
- **Version**: 1.7.19