sticky side bar

First, I have just installed Verbosa and I love it!

I am a complete newbie and haven’t modified anything with CSS and I have limited plugins.

I expected my sidebar and my post area to scroll separately but they stick together most of the time and then separate as I try to scroll up or down in an odd and unpredictable way.

How do I fix this? It is the only problem I have had but it makes the site very unattractive.

thank you!

Website: www.markpetertully.com