paint method
Paints the border within the given Rect on the given Canvas.
The textDirection argument must be provided and non-null if the border
has a text direction dependency (for example if it is expressed in terms
of "start" and "end" instead of "left" and "right"). It may be null if
the border will not need the text direction to paint itself.
Implementation
@override
void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {
// For some reason, when we paint the background in this shape directly, it
// lags while scrolling. Hence, we only use it to provide the outer path
// used for clipping.
}