Quick drop-in snippet to disable the default full-screen editing mode and welcome guide in WordPress when a user first visits the edit interface.

disable-fullscreen-snippet.php

<?php
function wp378934573289_js_head_print() {
	$screen = get_current_screen();

	// Only add script in editor views.
	if ( 'edit' !== $screen->parent_base ) {
		return;
	}

	?>

	<script>
		document.addEventListener( 'DOMContentLoaded', () => {

			/**
			* Feature statuses to enforce.
			*
			* $feature => $desiredState
			*
			*/
			let featureDeclaration = {
					'fullscreenMode' : false,
					'welcomeGuide'   : false
				},
				previousState = {};

			// Fetch any user-defined preferences.
			const userPreference = localStorage.getItem( 'featureStatusOverrides' );
			if ( null !== userPreference ) {
				featureDeclaration = JSON.parse( userPreference );
			}

			// Loop over the available features that are defines as overrideable.
			for ( const [ key, value ] of Object.entries( featureDeclaration ) ) {
				previousState[ key ] = value;

				// If the current editor state for the feature does not match what we expect, toggle the feature state.
				if ( value !== wp.data.select( 'core/edit-post' ).isFeatureActive( key ) ) {
					wp.data.dispatch( 'core/edit-post' ).toggleFeature( key );
				}
			}

			// Subscribe to feature changes in the editor.
			wp.data.subscribe( () => {
				let currentState;

				// Loop over our controlled featurestot see if any of them have been set by the user.
				for ( const [ key, value ] of Object.entries( featureDeclaration ) ) {
					currentState = wp.data.select( 'core/edit-post' ).isFeatureActive( key );

					// If the preference has been changed for a setting, update the personal preferences.
					if ( previousState[ key ] !== currentState ) {
						previousState[ key ] = currentState;
					}
				}

				// Store the custom editor preferences for this user in localStorage for next time.
				localStorage.setItem( 'featureStatusOverrides', JSON.stringify( previousState ) );
			} );
		} );
	</script>

	<?php
}

add_action( 'admin_print_footer_scripts', 'wp378934573289_js_head_print', 50 );