(In 3.6, TreeWidget.py became tree.py.)
Even though IDLE trees now appear to work correctly, including on Ubuntu, I am concerned about possible memory leaks, which are usually detected by repeatedly running a module's test file.  Test_tree does not leak now, but it does not do much. In particular, it does not expand and contract items and move selections.  TreeNode.draw has this note: "This leaks bindings until canvas is deleted", and I wonder if the same could be true in .drawtext.

Tree.py currently has these notes at the top:
# - keep track of object ids to allow more careful cleaning
# - optimize tree redraw after expand of subnode
Currently, self.text_id is saved, but not used.  Reusing canvas items should be more efficient.

I won't edit tree.py immediately, as I intend to try using ttk.Treeview as a replacement.  But we might instead update tree.py with ttk components.  In the meanwhile, I will leave this open.