The Index Page which includes both the side navigation and Discussion List should be wrapped in a container as Flarum utilizes Bootstrap in it's design.
Are you using any additional CSS/LESS? Have any themes installed?
I have noticed that on laptops (such as the Yoga C940) if you have the "Change the size of text, apps, and other items" Display option set to anything above 100% it will make websites seem much wider than they really are. Sometimes these settings are set by default by the OEM.