On a computer, the sidebar now starts out of the way. Pages take the full width, and the navigation opens as a panel from the menu button at the left of the page bar.
Select Keep sidebar open in the panel to put the sidebar beside the page. Magpie remembers this in that browser. Select Collapse sidebar to go back to the panel.
If you had not chosen either before, your sidebar now starts as the panel too. Select Keep sidebar open once to bring it back.