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.
User:Fanoflionking | |||||
|---|---|---|---|---|---|
| Pages created |
| ||||
| User pages | |||||
| User talk | |||||
| landmark edits | |||||
| Awards | |||||
| Others | |||||