paint method
- @override
 
override
    Paint this input border on canvas.
The rect parameter bounds the InputDecorator's container.
The additional gap parameters reflect the state of the InputDecorator's
floating label. When an input decorator gains the focus, its label
animates upwards, to make room for the input child. The gapStart and
gapExtent parameters define a floating label width interval, and
gapPercentage defines the animation's progress (0.0 to 1.0).
Implementation
@override
void paint(Canvas canvas, Rect rect, {
    double gapStart,
    double gapExtent = 0.0,
    double gapPercentage = 0.0,
    TextDirection textDirection,
});