When using a single value field, user can select multiple items and after form validation an error is printed. We can improve user interaction by handling fancy tree behavior according to field storage config.
Add a term field with cardinality set to 1, and try to add more than to 1 term, see the printed error.
Use the selectMode & checkbox fancy tree options to display radios instead of checkboxes in this case.
Use the patch attached to this issue to solve this problem, no configuration needed ;)
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