Sidebar to scroll

Hello,
after a long struggle I tried to make a page. But I found out a problem which I cannot solve. I hope I can have some support here:

I′ve created a sidebar with the widget “custom menu”. Every link there is one page.
Because of the high amount of pages, the sidebar expands and gets longer.
Is there a possibility, to keep the sidebar somehow in a frame and eventually scroll within? And it should stay marked at the current page.
An example of how it could look like you will find below:

Thank You

The page I need help with: [log in to see the link]