That's not true as if you just reduce the height of the browser the content is unreachable. I usually tile my windows to work on more than one thing at once so I never ever have my browser maximised. It's a minor issue but an annoying one for me
This sidebar right now is dynamic, it moves dynamically. But I want to do is to make positioned absolutely/fixed and when you click/select menu category, only the content will scroll.
The problem is that this is heavily scripted; it is not a CSS solution as such and will need help from the JS forum so I will move the thread there.
If you just wanted the header fixed and the sidebar fixed we could do that with css but then you would never be able to reach the sidebar content because it is below the fold even when my browser is maximised. It seems you must have the dynamic (js) solution for the design that you have to work.
You also say you want the header fixed but it depends on what you are calling the header? You would be better of just creating a sticky nav at the top as in that demo I showed. Of course this is JS territory again and not my remit.
You need someone like @Pullo to advise or one of the other JS wizards.