Index: src/wp-admin/js/widgets.js
===================================================================
--- src/wp-admin/js/widgets.js	(revision 31848)
+++ src/wp-admin/js/widgets.js	(working copy)
@@ -135,6 +135,8 @@
 					// Prevents jumping when dragging a widget from an open sidebar to a closed sidebar below.
 					height = ui.item.hasClass('ui-draggable') ? $this.height() : 1 + $this.height();
 					$this.css( 'min-height', height + 'px' );
+				} else {
+					$wrap.removeClass('closed');
 				}
 			},