Fix block d&d with `dragContent`

src/block_manager/view/BlockView.js

@@ -4,7 +4,8 @@ import { on, off, hasDnd } from 'utils/mixins';
 module.exports = Backbone.View.extend({
   events: {
     mousedown: 'startDrag',
-    dragstart: 'handleDragStart'
+    dragstart: 'handleDragStart',
+    dragend: 'handleDragEnd'
   },
 
   initialize(o, config = {}) {
@@ -36,8 +37,15 @@ module.exports = Backbone.View.extend({
     const isObj = isObject(content);
     const type = isObj ? 'text/json' : 'text';
     const data = isObj ? JSON.stringify(content) : content;
-    // Note: data are not available on dragenter for security reason
+
+    // Note: data are not available on dragenter for security reason,
+    // but will use dragContent as I need it for the Sorter context
     ev.dataTransfer.setData(type, data);
+    this.em.set('dragContent', content);
+  },
+
+  handleDragEnd() {
+    this.em.set('dragContent', '');
   },
 
   /**

src/utils/Droppable.js

@@ -82,8 +82,11 @@ export default class Droppable {
       },
       document: canvas.getFrameEl().contentDocument
     });
-    const content = this.getContentByData(dt).content || '<br>';
-    this.sorter.setDropContent(content); // should not be empty
+    // For security reason I can't read the drag data on dragenter, but
+    // as I need it for the Sorter context I will use `dragContent` or just
+    // any not empty element
+    const content = em.get('dragContent') || '<br>';
+    this.sorter.setDropContent(content);
     this.sorter.startSort(this.el);
     em.trigger('canvas:dragenter', dt, content);
   }