Gutenberg disable fullscreen as the default editor

disable-fullscreen.js

/**
 * This checks if the fullscreen mode is "on" on page load and toggle it off.
 */
(function (wp) {
    const isFullscreenMode = wp.data.select('core/edit-post').isFeatureActive('fullscreenMode');
    if (isFullscreenMode) {
        wp.data.dispatch('core/edit-post').toggleFeature('fullscreenMode');
    }
})(window.wp);

wp-disable-gutenberg-fullscreen.php

<?php

/**
 * Toggle off the fullscreen when editing a post. This can be toggled on later.
 */
public function tim_disable_default_fullscreen_editor()
{
    // enqueue script with 'wp-blocks' will ensure that the blocks are loaded before calling the script
    wp_enqueue_script('tim-disable-fullscreen', stylesheet_directory_uri().'js/disable-fullscreen.js', ['wp-blocks']);
}

// this hook is callon only when you are using Gutenberg editor (no other admin page)
add_action('enqueue_block_editor_assets', 'tim_disable_default_fullscreen_editor');