"Scroll to most recent messages" button is pixellated on both element-web and element-desktop

### Description

The "Scroll to most recent messages" button is heavily pixellated on both element-web and element-desktop. Although a different button, this is not present on mobile (mobile Android, at least).

### Steps to reproduce

- Navigate to app.element.io, sign in, scroll up, and observe.
- Or, install element-desktop, open up a room, scroll up, and observe.

Left is element-desktop, right is element-web (in Chromium)

### Version information

- **Platform**: web and desktop
- **Browser**: Ungoogled Chromium 87
- **OS**: EndeavourOS Linux 5.9.14
- **URL**: app.element.io
- **Version**: 1.7.14