On some websites, the vertical sidebar has a dark background. As a result, the also dark scrollbar thumb becomes invisible. Opening the same site in Safari does not have this issue.
Example from https://github.com/jaredsinclair/circular-buffer/blob/master/Sources/CircularBuffer/CircularBuffer.swift:

I don't use any extensions in Orion at all (yet! 😉), and I've set the macOS scrollbar setting to always show.
Alas, I have no idea what steps could cause this issue. Please try opening the link from the above text field at your end, and see if you see the same issue.
Sufficient contrast between scrollbar background and thumb so it's always possible to a) see the current position of the content in the viewport, and b) know where to grab the thumb for moving that content in the viewport.
Orion 1.0.4; macOS 26.3.1 (a)
Tahoe (26)