Unread marker in timeline (Green line) appeared in odd places

### Steps to reproduce

The green line that appears in the timeline for unread messages appeared:
- In the room list.
- At the bottom of the timeline, below all the messages.
- On top of my own message.
- - It's probably still counting hidden messages.

### Outcome

#### What did you expect?
For it not to be there
#### What happened instead?
It was there.

### Operating system

Linux Mint

### Application version

Element Nightly version: 2026082401

### How did you install the app?

elelment.io/get-started

### Homeserver

matrix.org

### Will you send logs?

Yes

Reproduce the unread marker behavior in Element Nightly on Linux Mint using matrix.org, following the listed timeline and room-list cases. Trace the timeline unread-marker entry point and verify that the green line no longer appears in the room list, below all messages, or on top of the user's own message, including when hidden messages are present.

Written by the indexing model from the issue text.