This is a relatively small honestly nitpicky, purely visual bug.
Steps to reproduce:
When you're on the Facebook Messenger browser page at www.messenger.com, scroll down on an active chat, where you will find that you can scroll down past the end of the chat leaving a gray space at the bottom of the screen.
This issue still occurs on Orion on compatibility mode. It also does not occur on Safari.
I would expect to not be able to scroll past the end of the chat leaving that blank space there. In order to make it disappear you have to rescroll up. As I mentioned before this is purely nitpicky and in the scheme of things not a very big deal.
Orion, OS version; hardware type:
Orion Version 0.99.116-beta (WebKit 614.1.20)
macOS Monterey 12.3.1
Macbook Pro 14 inch, 2021