This gadget option (for Vector skin) shows the desktop page in a mobile view sidebar while browsing the desktop site.

Enable this mobile view in your Gadgets section of your Preferences. Check the box for: Mobile sidebar preview - Show page in mobile view while browsing the desktop site, then scroll to bottom of page and save your changes.

There is a Smartphone shaped icon located in the top bar of tabs. Press the Smartphone icon to toggle the mobile view on and off. The expanded screenshot here is highlighted with a red box and arrow for illustrative purposes.

You can turn off the mobile view by pressing the white-X in the black button at the bottom of the Smartphone frame. The Smartphone frame includes its own scrollbar independent of the Desktop scrollbar.