Fully read marker is set at the bottommost message even if no message text is visible

### Steps to reproduce

1. Find a room with enough not-fully read messages to fill a screen. Move to the fully read marker.
2. Scroll down the timeline so that just the author name of (some) not fully read message is visible (see the screenshot).

### Outcome

#### What did you expect?
The previous message is marked as fully read, not the one that only has the author name on the screen.

#### What happened instead?
The lowest technically-visible event is marked as fully read. Technical visibility does not coincide with common-sense visibility in this case: it's impossible to actually _read_ even the first line of that message.

It might make sense to mark the message as fully read only when its bottom edge is displayed, maybe?

### Operating system

Fedora Linux 38

### Browser information

Firefox 116.0.3

### URL for webapp

staging.element.io

### Application version

Element version: 1.11.40-rc.1

### Homeserver

matrix.org

### Will you send logs?

No