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New tests should always use only one Tk window at once, like all the
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current tests do. This means that you have to destroy the current window
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before creating another one, and clean up after the test. The motivation
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behind this is that some tests may depend on having its window focused
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while it is running to work properly, and it may be hard to force focus
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on your window across platforms (right now only test_traversal at
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test_ttk.test_widgets.NotebookTest depends on this).