Can't scroll below certain message in timeline

### Steps to reproduce

1. Open a room with lots of unread messages (to ensure there's enough for the bug to occur).
2. Jump to the first unread message.
3. Start scrolling down.

### Outcome

#### What did you expect?

To eventually reach the bottom of the timeline.

#### What happened instead?

At a certain point while scrolling down, the timeline jumps up by several messages. Once you hit this bug, every time you scroll back down to that section of the timeline, you get thrown back up. So you're stuck in a loop (the distance you're punted back seems consistent).

This buggy section is different each time the bug occurs (I've only had it happen once per session). You can't recreate it with an old read section where the bug occurred previously, only with new unread ones.

If you try to jump to the bottom and scroll back up, the moment you reach near that section, you're thrown back up the timeline again. Also, it's hard to tell whether you have to reach a specific message, or an entire section of the timeline.

The buggy behaviour can be removed by simply reloading the page.

### Operating system

_No response_

### Browser information

Firefox 115.1.0esr

### URL for webapp

_No response_

### Application version

Element 1.11.40, olm 3.2.14

### Homeserver

_No response_

### Will you send logs?

No