I would recommend to use z-index:100; in the css style of the navigation container. by setting the z-index, you basically 'lift-up' the navigation container from the rest of the page.
Hope this will help,
If you go to the layout of your page, you can select how the sidebar should behave in the properties of the sidebar:
When you have selected, set the toggle mode to none and it will always stay active. Please make sure you also remove the toggle button as that doesn't have a use anymore.
But I don't want it to be always active, I want the sidebar to appear when I click on the button, but not to push the content of the page, but to stay on top of it.
Like this for example: