code: function toggleToolbarsScrollableOnResize() {
  		function toggleToolbarsScrollableClass() {
  			editor.$( '.mce-toolbar-grp', editor.theme.panel.getEl() ).each( ( i, toolbar ) => {
  				const isScrollable = toolbar.scrollWidth > toolbar.clientWidth;
  				editor.$( toolbar ).toggleClass( 'is-scrollable', isScrollable );
  			} );
  		}
  
  		window.addEventListener( 'resize', throttle( toggleToolbarsScrollableClass, 200 ) );
  		toggleToolbarsScrollableClass();
  
  		// Since some toolbars are hidden by default and report inaccurate
  		// dimensions when forced to be shown, we instead bind to the event
  		// when it's expected that they'll be visible
  		editor.on( 'wptoolbar', event => {
  			// Since an event handler is expected to set the toolbar property,
  			// set a timeout to wait until the toolbar has been assigned
  			setTimeout( () => {
  				if ( ! event.toolbar || ! event.toolbar.visible() ) {
  					return;
  				}
  
  				// For inline toolbars, the scrollable panel is the first child
  				// of the toolbar, so we compare its width against the parent
  				const toolbar = event.toolbar.getEl();
  				const isScrollable = toolbar.firstChild.scrollWidth > toolbar.clientWidth;
  				editor.dom.toggleClass( toolbar, 'is-scrollable', isScrollable );
  			}, 0 );
  		} );
  	}
docs: Monitors the window resize event, assigning an `.is-scrollable` class to
  any toolbars for which horizontal scroll exists.