Right side panel doesn't disappear when browser gets smaller in width

E.g. https://doc.qt.io/squish/squish-api.html when browser is fullscreen, I see a right side panel with navigation on this page. When making the window less wide, the navigation disappears but the right panel stays (and thus is empty). This then makes the main content smaller then necessary, on the link mentioned, the function overview is gets scrolled away.

So I think when the right navigation disappears, the space should be given to the main content.