If you just started using WordPress you probably noticed that everytime you’re creating a new post or page Gutenberg editor is open in Fullscreen mode. There are no menus on the left, and for some it might be quite annoying.
You can disable the Fullscreen mode in every single post or page manually, but it’s not ideal solution. Here’s how you can permanently disable Fullscreen mode in WordPress editor by default.
How to automatically disable Fullscreen Mode in WordPress editor
The easiest and most straightforward way to automatically disable Fullscreen mode in WordPress Gutenberg editor is to install plugin that does exactly that.
Go to Plugins >> Add New, and enter Fullscreen mode B gone (plugin link) to the search field at the top right of the page. Search results will show you the extension, and everything what’s left will be to install and activate it.

After activating the plugin Fullscreen Mode will be disabled by default.
You could enable it again on individual posts (or pages) in the post’s More Tools and options tab.

That’s it – now all the new posts and pages won’t have Fullscreen mode enabled unless you do so.