While editing Pages in Site Builder, your default view is a "Quick Preview." This view includes the editing tools and navigation elements on the left/top of your screen.
While this view is a good way to get a sense of how your design is progressing, you should periodically check your progressive in the full desktop of mobile preview.
To Preview Your Website in Full Desktop Preview:
From Site Builder, select Preview from the drop down menu.
A new browser window/tab will open. This screen displays the website just as your website visitors will see it on a desktop computer.