wp-admin.css class="hidden" bug renders screen options menu in admin blank

One of the publishers who uses our GrabPress plugin brought a serious issue to our attention. Initially they thought it was a bug with our plugin, but after looking further into the issue, it’s clear that the issue is within the WordPress admin files.

The Screen Options menu at the top of the WordPress admin dashboard becomes unusable, and the form elements appear hidden. Reproducing this issue was difficult to track, mainly because it just depends on when other CSS files conflict with it (which can be many). It leaves the ‘.hidden’ class out there to potentially conflict against any other CSS file in the environment that uses the ‘visibility:hidden’ property instead of or in addition to ‘display:none’

One of our developers took a closer look and found the following.

In the PHP file where the original state of the HTML for the screen options UI that is experiencing the issue is formed you’ll see that the class=”hidden” is present.
wp-admin/includes/screen.php (line 917)

<div id="screen-options-wrap" class="hidden" tabindex="-1" aria-label="<?php esc_attr_e('Screen Options Tab'); ?>">

They have this class tied up to the following CSS:
wp-admin/css/wp-admin.css (line 234 thru 243)

.hidden,
.js .closed .inside,
.js .hide-if-js,
.no-js .hide-if-no-js,
.js.wp-core-ui .hide-if-js,
.js .wp-core-ui .hide-if-js,
.no-js.wp-core-ui .hide-if-no-js,
.no-js .wp-core-ui .hide-if-no-js {
	display: none;
}

So you can see that the ‘.hidden’ class (along with several others) has the property ‘display: none;’. This is perfectly fine and normal if they plan to toggle on and off the ‘.hidden’ class from the element. However they instead make use of jQuery’s ‘show()’ and ‘hide()’ methods and leave the ‘hidden’ class.
wp-admin/js/common.js (lines 99 thru 143)

screenMeta = {
	element: null, // #screen-meta
	toggles: null, // .screen-meta-toggle
	page:    null, // #wpcontent

	init: function() {
		this.element = $('#screen-meta');
		this.toggles = $('.screen-meta-toggle a');
		this.page    = $('#wpcontent');

		this.toggles.click( this.toggleEvent );
	},

	toggleEvent: function( e ) {
		var panel = $( this.href.replace(/.+#/, '#') );
		e.preventDefault();

		if ( !panel.length )
			return;

		if ( panel.is(':visible') )
			screenMeta.close( panel, $(this) );
		else
			screenMeta.open( panel, $(this) );
	},

	open: function( panel, link ) {

		$('.screen-meta-toggle').not( link.parent() ).css('visibility', 'hidden');

		panel.parent().show();
		panel.slideDown( 'fast', function() {
			panel.focus();
			link.addClass('screen-meta-active').attr('aria-expanded', true);
		});
	},

	close: function( panel, link ) {
		panel.slideUp( 'fast', function() {
			link.removeClass('screen-meta-active').attr('aria-expanded', false);
			$('.screen-meta-toggle').css('visibility', '');
			panel.parent().hide();
		});
	}
};

The use of these methods is also okay. The ‘show()’ and ‘hide()’ methods toggle the element’s display value between ‘display:none’ and ‘display:block’. The issue here is that WP used the ‘.hidden’ class to set the default display state and then use the Jquery toggle methods to adjust on the fly. However, this leaves the ‘.hidden’ class out there to snag itself in conflict against any CSS at play that uses the ‘visibility:hidden’ property instead of or in addition to ‘display:none’. Twitter Bootstrap for example, which GrabPress uses and is widely used across the web has the following declaration for ‘.hidden’:

.hidden {
  display: none;
  visibility: hidden;
}

So my recommendation to WP would be to have their JavaScript toggle on/off the class ‘hidden’ from the element rather than leaving it there. To have ‘hidden’ on a unhidden element just makes no sense, even semantically speaking. It might also be good for them to toggle both the ‘display’ property and the ‘visibility’ property of the element.