ui.dialog.overlay.js

_createOverlay: function() {
		if ( !this.options.modal ) {
			return;
		}

		// We use a delay in case the overlay is created from an
		// event that we're going to be cancelling (#2804)
		var isOpening = true;
		this._delay(function() {
			isOpening = false;
		});
		
		var countOverlays = ( this.document.data( "ui-dialog-overlays" ) || 0 );

		if ( 0 === countOverlays ) {

			// Prevent use of anchors and inputs
			// Using _on() for an event handler shared across many instances is
			// safe because the dialogs stack and must be closed in reverse order
			this._on( this.document, {
				focusin: function( event ) {
					if ( isOpening ) {
						return;
					}

					if ( !this._allowInteraction( event ) ) {
						event.preventDefault();
						this._trackingInstances()[ 0 ]._focusTabbable();
					}
				}
			});
		}
		
		this.overlay = $( "<div>" ).addClass( "ui-widget-overlay ui-front" );
		
		if ( countOverlays > 0 ) {
			this.overlay.css('z-index', +this.overlay.css('z-index') + countOverlays)
		}
		this.overlay.appendTo( this._appendTo() );
		this._on( this.overlay, {
			mousedown: "_keepFocus"
		});
		this.document.data( "ui-dialog-overlays",(countOverlays + 1));
	}