The number of pixels to offset the children in the opposite of the axis direction.
For example, if the axis direction is down, then the pixel value represents the number of logical pixels to move the children up the screen. Similarly, if the axis direction is left, then the pixels value represents the number of logical pixels to move the children to right.
This object notifies its listeners when this value changes (except when the value changes due to correctBy).
@override double get pixels => _pixels;