How can I fix the header in the modal window if its width is in percent?

Question

css

https://jsfiddle.net/sharomet/w9d9d0fL/
Hello everyone.
Such a question, for example, I have a moral window like in bootstrap (the example just shows a block with a width of %)
The bootstrap modal has a header And I want to make it scroll along with the whole page since the window contains a lot of information and in the header control buttons. In my case, the width of the window as a percentage.
How can I do that?
Thank you.

Answer 1

https://jsfiddle.net/6s3d1yzx/