MediaWiki:Gadget-HorizontalNavigation.js

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const HorizontalNavigation = {

	init( $content ) {
		$content.find( '.template-horizontal-navigation caption' ).on( 'click', HorizontalNavigation.toggle );
	},

	toggle( event ) {
		if ( event.target.tagName === 'A' ) {
			return;
		}
		$( this ).next( 'tbody' ).toggle();
	},
};

mw.hook( 'wikipage.content' ).add( HorizontalNavigation.init );