Dialog: Fix stacking of modal dialog overlays

Fixes #10138
Closes gh-1286

Author: scottgonzalez

ui/dialog.js

@@ -260,6 +260,14 @@ return $.widget( "ui.dialog", {
 		this._position();
 		this._createOverlay();
 		this._moveToTop( null, true );
+
+		// Ensure the overlay is moved to the top with the dialog, but only when
+		// opening. The overlay shouldn't move after the dialog is open so that
+		// modeless dialogs opened after the modal dialog stack properly.
+		if ( this.overlay ) {
+			this.overlay.css( "z-index", this.uiDialog.css( "z-index" ) - 1 );
+		}
+
 		this._show( this.uiDialog, this.options.show, function() {
 			that._focusTabbable();
 			that._trigger( "focus" );