The sidebar extension takes me directly to Facebook. I can seemingly do usual Facebook things except for Messenger, and attempting to use it will just redirect me to a separate tab which just defeats the purpose of this extension.
I'm using 71.0.3770.271 version; if I'm not mistaken this is the latest version of Opera. Prior to encountering this problem I had cleared my cookies, which unsurprisingly signed out all my accounts. However, I was forced to use the updated Messenger, the one with pink/blue/whatever colors. It was here that I was no longer able to use the extension properly.
I intend to enable
opera://flags/#sidebar-site-panel as a temporary fix, though I have never tried this before and have only picked this up from other forum entries.