For a single checkbox there is always value true/false (That's ok). But in such case field is always 'correct' even if the field is not check - for condition required = yes. The behavior is not obvious and a little confusing.
The only fix is to use formulas required like "./text() = 'false' " but in the result after the field is checked star disappears.
Re: single checkbox - required: what does it mean?
Yes, I agree. Should making a Single checkbox "required" be "mapped" to a
constraint `. = 'true'` in the source? I reckon this would make sense.
Thinking out loud, the thing is that Form Builder doesn't have a knowledge
of specific XBL components, so we would need a mechanism for XBL components
to declare that required is enforced using a constraint they provide. When
users mark the filed as required, Form Builder would add the constraint, and
conversely, when it finds that constraint, it would indicate the field as
required in the Control Settings. Maybe inside: