Expanding hidden events does not increase the scrollable area in the timeline, causing one to be unable to read the whole event

### Description

With show hidden events enabled, if I am in a room where there's not much space to scroll back, and open a hidden event with a considerable size, I may be unable to view the entire event as it doesn't seem like the scroll of the timeline takes the size of the expanded hidden event into account.

Video of the behaviour: https://streamable.com/r2r8k8

I understand this is a dev feature, just documenting for posterity :)

### Steps to reproduce

- Create a room
- Make a call or do something else that generates a big hidden event (this could also be a problem with normal hidden events and a small screen size
- Expand the hidden event (or possibly just expand multiple hidden events in a room with little scrollback)
- Try to scroll to view the whole event

Logs being sent: no

### Version information

- **Platform**: desktop
- **OS**: Manjaro Linux
- **Version**: Element Nightly 2020101701

Reproduce the issue in the timeline using a large hidden event, then inspect the timeline and hidden-event expansion entry points involved in calculating the scrollable area. Done means an expanded hidden event, or multiple expanded hidden events, can be scrolled through completely even when the room has little scrollback.

Written by the indexing model from the issue text.