currently both "Visible To" and "Editable By" fields are normal dropdown select fields in Maniphests Create and Edit task view.
Use case: I have tens of projects with their own access policies. I have to visually find the project in the dropdown list and I alway have to scroll the entire page down. I have to do this twice, because both fields need to be set.
If boths fields would be a searchable combobox instead, then changing them would be easier and faster.