At any given time, the editor gives you the possibility to preview the website so you can have a global picture of what you are designing and test tools that don't show while building the pages, e.g the Fix Header to Top option.
How to preview the website
You will find the Preview option in the upper right corner of the editor. Once you click on it, a new tab with the preview of your website will appear in the different viewports.
Feel free to scroll down and explore the website, including testing some elements. If you wish to preview the website full-screen on your desktop, simply click on "Remove Frame" in the top right corner.
Once you have seen all you have wanted, you can close the tab and keep working on the editor.
Good to Know
If you want to avoid opening the Preview with the different viewports (desktop, mobile and tablet view), hold down the ALT key in your keyboard when clicking "Preview" on the editor. The editor will then open a preview of your website only on the desktop version.