Tree: address some draggable issues (#10494)

packages/tree/src/tree.vue

@@ -334,6 +334,13 @@
           return false;
         }
         event.dataTransfer.effectAllowed = 'move';
+
+        // wrap in try catch to address IE's error when first param is 'text/plain'
+        try {
+          // setData is required for draggable to work in FireFox
+          // the content has to be '' so dragging a node out of the tree won't open a new tab in FireFox
+          event.dataTransfer.setData('text/plain', '');
+        } catch (e) {}
         dragState.draggingNode = treeNode;
         this.$emit('node-drag-start', treeNode.node, event);
       });