The coverage of Phil's patch is about 60%. I decided to push it with slight modification so he is properly credited with what he did. Changes: 
Move mock AutoCompleteWindow to test file; too special.
Move ac_func to mock_idle for use in other tests; well done!
Redo Event mock. Assign needed attributes when create.
Make changes needed because of above two changes.
Add macosxSupport call now needed to run without raising.
Delete root when done.
Add abbreviation for self.autocomplete.open_completions, etc.
Improve the logic of a couple of subtests.

Any blank class could be used to mock event, but putting it in mock_tk allows for a docstring and does the update trick just once.

Saimadhav, open a new issue to work more on this test.