Function to add to functions.php that will toggle the Fullscreen editor off on the WP Admin.

wp-disable-fullscreen-editor.php

<?php
if (is_admin()) { 
    function pa_disable_fullscreen_wp_editor() {
        $script = "jQuery(window).load(function() { const isFullscreenMode = wp.data.select('core/edit-post').isFeatureActive('fullscreenMode'); if (isFullscreenMode) { wp.data.dispatch('core/edit-post').toggleFeature('fullscreenMode'); } });";
        wp_add_inline_script('wp-blocks', $script);
    }
    add_action('enqueue_block_editor_assets', 'pa_disable_fullscreen_wp_editor');
}