CiviCase: editing custom data twice in CaseView acts weirdly

In CiviCase,

• create a custom field group that applies to Cases (with at least one field)
• create a case
• in CaseView, click the "modify" button to edit the custom fields (open a popup)
• click "Cancel" or press "Escape" to not save
• try to edit another time: it will popup a window, but then fade away.