Tree editor: incorrect selection behaviour after window loses and re-acquires focus

If a row is selected in a tree editor, then losing and re-gaining focus and clicking on a new row seems to break the display of selected rows until the original row is clicked again.

See the animation below, which uses the `traitsui/examples/demo/Standard_Editors/TreeEditor_demo.py` file of this repo.

I would have expected the row called "Scientific" to be unselected as soon as the "Dave" row is clicked.