sisable-gutenberg-fullscreen-mode.js

/*
Title:     Disable Gutenberg fullscreen mode
Type:      JavaScript > Custom Code
Location:  Custom Hooks
Hook name: enqueue_block_editor_assets
Priority:  10
*/

window.addEventListener('load', function() {
    if (wp.data.select('core/edit-post').isFeatureActive('fullscreenMode')) {
        wp.data.dispatch('core/edit-post').toggleFeature('fullscreenMode');
    }
});