Add transferEditSessionAtIndex to Editor

src/app/editor.coffee

@@ -549,6 +549,9 @@ class Editor extends View
       "Cancel"
     )
 
+  transferEditSessionAtIndex: (fromIndex, toIndex, toEditor) ->
+    toEditor.editSessions.splice(toIndex, 0, @editSessions.splice(fromIndex, 1)[0])
+
   activateEditSessionForPath: (path) ->
     for editSession, index in @editSessions
       if editSession.buffer.getPath() == path

src/packages/tabs/lib/tab-view.coffee

@@ -89,7 +89,7 @@ class TabView extends SortableList
     draggedTab.remove()
     draggedTab.insertAfter(droppedNearTab)
     currentDraggedTabIndex = draggedTab.index()
-    toEditor.editSessions.splice(currentDraggedTabIndex, 0, fromEditor.editSessions.splice(previousDraggedTabIndex, 1)[0])
+    fromEditor.transferEditSessionAtIndex(previousDraggedTabIndex, currentDraggedTabIndex, toEditor)
 
     if !fromPane.find('.tab').length
       fromPane.view().remove()