the "Full width" button at the top-left corner of the webapp has no effect for viewport widths between 662px and 948px (these numbers may vary from one computer to the other).

It should either be hidden when it has no to little effect (ie. when under under 383px, or when slightly over 383px, 512px, or 948px) and/or get a tooltip (and/or be removed and always considered switched on, IMO)