I think the screenshot is enough to describe it
1Password Version: Not ProvidedExtension Version: 1.15.4OS Version: Ubuntu 18.04Sync Type: Not Provided
Sorry, I'm not sure I'm really getting why it's horrible. The scrollbar style is dictated by the operating system so we're not explicitly setting it like that. The menu area is a fixed size set prior to loading any items and from a previous discussion I was told dynamic resizing isn't pleasant or something along those lines. My takeaway from that was user re-sizeable or dynamic resizing were unlikely.
It's quite possible I haven't understood though so please do correct me if you feel that's the case.
Just to show what Adam mean, here's a scrollbar on macOS:
However, the design team is looking into this for sure. We will try to find a way to make it more inline (not clipping top-right corner for an example) and prettier.