Defines the ink response focus, hover, and splash colors.
This default null property can be used as an alternative to focusColor, hoverColor, and splashColor. If non-null, it is resolved against one of MaterialState.focused, MaterialState.hovered, and MaterialState.pressed. It's convenient to use when the parent widget can pass along its own MaterialStateProperty value for the overlay color.
MaterialState.pressed triggers a ripple (an ink splash), per the current Material Design spec. The overlayColor doesn't map a state to highlightColor because a separate highlight is not used by the current design guidelines. See https://material.io/design/interaction/states.html#pressed
- The Material Design specification for overlay colors and how they match a component's state: material.io/design/interaction/states.html#anatomy.
final MaterialStateProperty<Color> overlayColor