Have a fixed position div that needs to scroll if content overflows

The problem with using height:100% is that it will be 100% of the page instead of 100% of the window (as you would probably expect it to be). This will cause the problem that you're seeing, because the non-fixed content is long enough to include the fixed content with 100% height without requiring a scroll bar. The browser doesn't know/care that you can't actually scroll that bar down to see it

You can use fixed to accomplish what you're trying to do.

.fixed-content {
    top: 0;
    bottom:0;
    position:fixed;
    overflow-y:scroll;
    overflow-x:hidden;
}

This fork of your fiddle shows my fix: http://jsfiddle.net/strider820/84AsW/1/

The solutions here didn't work for me as I'm styling react components.

What worked though for the sidebar was

.sidebar{
position: sticky;
top: 0;
}

Hope this helps someone.