Panels are a useful feature that can show you the mobile or desktop view of any web page in the sidebar.
If the panels are not visible, click the panel toggle button on the status bar.
Click the plus (+) button to add the current page as a panel. At this point you can also modify the address to whatever you want.
You can also add any page as web panel if you right click into the page and choose Add page to web panel from the context menu.