Index: wp-admin/includes/class-wp-upgrader.php
===================================================================
--- wp-admin/includes/class-wp-upgrader.php	(revision 25726)
+++ wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -1697,6 +1697,7 @@
 			case 'plugin':
 				$plugin_data = get_plugin_data( $context . '/' . $item );
 				$item_name = $plugin_data['Name'];
+				$reactivate_flag = is_plugin_active( $item );
 				$skin->feedback( __( 'Updating plugin: %s' ), $item_name );
 				break;
 			case 'language':
@@ -1722,6 +1723,11 @@
 			}
 		}
 
+		// Reactivate the plugin if it was active before
+		if ( 'plugin' == $type && $reactivate_flag && !is_plugin_active( $item ) ) {
+			activate_plugin( $item, null, false, true );
+		}
+		
 		self::$upgrade_results[ $type ][] = (object) array(
 			'item'     => $item,
 			'result'   => $upgrade_result,