function activateOverview() {

		// Only proceed if enabled in config
		if( config.overview ) {

			// Don't auto-slide while in overview mode
			cancelAutoSlide();

			var wasActive = dom.wrapper.classList.contains( 'overview' );

			// Vary the depth of the overview based on screen size
			var depth = window.innerWidth < 400 ? 1000 : 2500;

			dom.wrapper.classList.add( 'overview' );
			dom.wrapper.classList.remove( 'overview-deactivating' );

			clearTimeout( activateOverviewTimeout );
			clearTimeout( deactivateOverviewTimeout );

			// Not the pretties solution, but need to let the overview
			// class apply first so that slides are measured accurately
			// before we can position them
			activateOverviewTimeout = setTimeout( function() {

				var horizontalSlides = document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR );

				for( var i = 0, len1 = horizontalSlides.length; i < len1; i++ ) {
					var hslide = horizontalSlides[i],
						hoffset = config.rtl ? -105 : 105;

					hslide.setAttribute( 'data-index-h', i );

					// Apply CSS transform
					transformElement( hslide, 'translateZ(-'+ depth +'px) translate(' + ( ( i - indexh ) * hoffset ) + '%, 0%)' );

					if( hslide.classList.contains( 'stack' ) ) {

						var verticalSlides = hslide.querySelectorAll( 'section' );

						for( var j = 0, len2 = verticalSlides.length; j < len2; j++ ) {
							var verticalIndex = i === indexh ? indexv : getPreviousVerticalIndex( hslide );

							var vslide = verticalSlides[j];

							vslide.setAttribute( 'data-index-h', i );
							vslide.setAttribute( 'data-index-v', j );

							// Apply CSS transform
							transformElement( vslide, 'translate(0%, ' + ( ( j - verticalIndex ) * 105 ) + '%)' );

							// Navigate to this slide on click
							vslide.addEventListener( 'click', onOverviewSlideClicked, true );
						}

					}
					else {

						// Navigate to this slide on click
						hslide.addEventListener( 'click', onOverviewSlideClicked, true );

					}
				}

				updateSlidesVisibility();

				layout();

				if( !wasActive ) {
					// Notify observers of the overview showing
					dispatchEvent( 'overviewshown', {
						'indexh': indexh,
						'indexv': indexv,
						'currentSlide': currentSlide
					} );
				}

			}, 10 );

		}

	}
Displays the overview of slides (quick nav) by
scaling down and arranging all slide elements.

Experimental feature, might be dropped if perf
can't be improved.