New-messages separator line isn't animated any more.

### Description
I hope this bug hasn't already been created, but I noticed that the separator line for new messages is not animated any more after reading the messages.

### Steps to reproduce
- Use the latest element-web/desktop version
- wait for new messages in a room
- see the green line, which is not animated
I was expecting the line to fade out animated (inward) as it did in previous versions. Instead, the line fades out suddenly, with no effect.

### Version information
- **Platform**: Both platforms (web and desktop) have this problem.

Web:
- **Browser**: Firefox 87.0
- **OS**: Fedora Linux
- **URL**: app.element.io

Desktop app:
- **OS**: Fedora Linux
- **Version**: 1.7.25