Auto-scroll on new messages is broken on certain monitor setups

### Description

I only notice this behaviour on my external monitor (1920x1080), but not on my Laptop screen (2560x1440). I find that:

* Scrolling to the bottom of room does not dismiss the "scroll-to-bottom" button in the bottom right corner.
* Sending a message does not scroll the room timeline to the bottom.

For example, I've used my scroll-wheel to scroll to the bottom of this room, but the scroll-to-bottom button is still showing, and still thinks there's a message hidden.


Moving the window to my Laptop screen and scrolling up and down dismisses the indicator.

I had a similar problem back in https://github.com/vector-im/element-web/issues/9342, though I'm not sure where the JS snipped went to test whether I'm having a non-int `scrollTop` value again.

Unlike the bug mentioned, this happens in all rooms.

Logs being sent: yes

### Version information

- **Platform**: desktop
- **OS**: Manjaro Linux
- **DE**: GNOME3
- **Version**: Element nightly 2020081601