How to stop navbar flickering when opening a modal window (Twitter Bootstrap)?

Question

css

When opening a modal window, on the page, under the window, the scroll disappears. Due to the change in the page width, the navigation menu adjusts to the new size, that is, it twitches, which is bad. At the same time, the rest of the elements on the page remain in their places.
Please tell me how to make the scroll on the page under the window not disappear, or just fix the width of the navigation menu?
Thank you!

Answer 1

I won’t throw off the finished solution right off the bat, I forgot it already, but when a modal window appears in the body tag, a class is prescribed that seems to be overflow-y: scroll should be assigned, and a similar scroll should be removed from the modal background.
See how the modal window in the second version of the bootstrap is done: getbootstrap.com/2.3.2/javascript.html and assign similar style properties to the modal window base and body. Will help.