Bug 467994 - drag and drop subfolder to root of account is blocked/fails

p=me r=mkmelin+mozilla sr=bienvenu

mail/base/content/folderPane.js

@@ -340,7 +340,12 @@ let gFolderTreeView = {
    */
   canDrop: function ftv_canDrop(aRow, aOrientation) {
     let targetFolder = gFolderTreeView._rowMap[aRow]._folder;
-    if (!targetFolder || !targetFolder.canFileMessages)
+    if (!targetFolder)
+      return false;
+    let dt = this._currentTransfer;
+    let types = dt.mozTypesAt(0);
+    if (Array.indexOf(types, "text/x-moz-message") != -1 &&
+        !targetFolder.canFileMessages)
       return false;
     if (aOrientation != Components.interfaces.nsITreeView.DROP_ON)
       return false;