Starting with WordPress 5.4, the editor behaves differently the first time you open the editor in a new installation or on a new device—or any other time WordPress resets the user preferences.

Now the editor opens in fullscreen mode by default. Note that for now, that’s a local setting, which is why it’s going to reset when your preferences do, including incognito mode. Future releases will store the setting in the WordPress database.

Want to turn it off? It’s simple—just use the pulldown in the editor’s menu.

You can also control the editor’s mode programmatically with the data module. A quick reminder: the code below is JavaScriptJavaScript JavaScript or JS is an object-oriented computer programming language commonly used to create interactive effects within web browsers. WordPress makes extensive use of JS for a better user experience. While PHP is executed on the server, JS executes within a user’s browser. https://www.javascript.com/., not PHPPHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 or higher.

const isFullscreenMode = wp.data.select( 'core/edit-post' ).isFeatureActive( 'fullscreenMode' );

if ( isFullscreenMode ) {
    wp.data.dispatch( 'core/edit-post' ).toggleFeature( 'fullscreenMode' );
}

#5-4, #block-editor, #dev-notes