I have been coding and re-coding my Tumblr theme for a week, and I am having the same problem over and over again.
Obviously, when my sidebar is not set to position:fixed; it doesn’t stay where I want it to. However, when it is set to that, my scrollbar stops working. If I set my sidebar to virtually any other position – relative, auto, initial, static – my scrollbar works just fine. I’ve tried setting my scrollbar to fixed, adding a z-index to it, nothing.
I have uploaded my CSS text here and you can see the problem here here.
The code dump in CodePen is a start, but unfortunately it doesn’t illustrate the problem (the shortcodes don’t mean much outside of Tumblr – you’d need to extract examples of the served HTML and put that and the CSS in the correct CodePen panels). What we need is a working demo. Something with just enough HTML and CSS to recreate/demo the problem you describe.