MarkdownWidgets should be able to scroll.

src/markdownwidget/plugin.ts

@@ -13,6 +13,8 @@ import {
   MarkdownWidgetFactory
 } from './widget';
 
+import './theme.css';
+
 /**
  * The class name for all main area portrait tab icons.
  */

src/markdownwidget/theme.css

@@ -0,0 +1,7 @@
+/*-----------------------------------------------------------------------------
+| Copyright (c) Jupyter Development Team.
+| Distributed under the terms of the Modified BSD License.
+|----------------------------------------------------------------------------*/
+.jp-MarkdownWidget {
+  overflow: auto;
+}