selected property
Whether this is the current one of its group — tri-state, and the third state is the point.
True when semantics says isSelected/isChecked/isToggled, false when
it says only hasSelectedState/hasCheckedState/hasToggledState, and
null when nothing said — which is different from false. Without the
distinction "not selected" and "not selectable" are the same answer, and
that difference is the whole value of the field.
Eight of the ten Material selection idioms publish it (measured);
SegmentedButton and hand-rolled InkWell tabs publish nothing, and for
those this stays null rather than being guessed from a colour.
Implementation
final bool? selected;