Sidebar Disappearing (How Do We Stop That?)

On a desktop computer, if you open up a post on such as and you maximize the screen, you will see a sidebar on the right hand side. If you slowly start decreasing the screen width, you will see the website change since it is responsive. However, at one point, the sidebar disappears and then reappears when the screen is made a little bit smaller (I'm attaching a screenshot to show you where the sidebar disappears).

How do we prevent the sidebar from disappearing at that screen width? We want the sidebar to remain visible until the window becomes some small that the sidebar has no room to be displayed along with the post text / images etc.

I have granted access via the wpmu support plugin, in case that is helpful.