Can't get sidebar fixed on the right side

Hi there,

I managed to display the side bar for my Twenty Fifteen Child Theme to the right side. And I tried through CSS to make this menu fixed (like it is when it is displayed in the original).

My CSS is :

sidebar {
float: right;
top: 0;
margin-right: 0px;
max-width: 413px;
position: fixed !important;
width: 29.4118%;
background: #fff;
box-shadow: 0 0 1px rgba(0, 0, 0, 0.15);
}

The problem I’m having is that when I type position: fixed, the whole sidebar moves back to its original position on the left side of the screen. When I use position: relative it stays on the right side but when I scroll down the page the sidebar stops before the content of the page ends.
I would like the sidebar to behave the same way as when it is displayed left.

Is there any way to do this through CSS or do I need to change things with PHP or Jscript as well?

Ellen