The biggest rectangle that is entirely inside the rounded rectangle and has the full width of the rounded rectangle. If the rounded rectangle does not have an axis-aligned intersection of its left and right side, the resulting Rect will have negative width or height.
Source
Rect get wideMiddleRect {
final double topRadius = math.max(tlRadiusY, trRadiusY);
final double bottomRadius = math.max(brRadiusY, blRadiusY);
return new Rect.fromLTRB(
left,
top + topRadius,
right,
bottom - bottomRadius
);
}