Jumping timeline and recurring unread marker at the bottom

### Steps to reproduce

Swtiching rooms caused the unread marker to reappear at the bottom and the timeline to glitch
https://github.com/user-attachments/assets/83139ca9-b80b-4ddf-833e-596d30fdbc3c

### Outcome

#### What did you expect?
No glitch.
No unread marker.
- No unread marker at the bottom of the chat.
#### What happened instead?
Glitch.
unread marker.
- unread marker at the bottom of the chat.

### Operating system

Linux mint

### Application version

Element Nightly version: 2026083101

### How did you install the app?

element.io/get-started

### Homeserver

matrix.org

### Will you send logs?

No

Start by reproducing the issue while switching rooms in Element Nightly on Linux Mint, focusing on the timeline and unread marker at the bottom of the chat. Compare the observed behavior with the attached video; done means the timeline no longer glitches and the unread marker does not reappear at the bottom.

Written by the indexing model from the issue text.