The biggest rectangle that is entirely inside the rounded rectangle and has the full height of the rounded rectangle. If the rounded rectangle does not have an axis-aligned intersection of its top and bottom side, the resulting Rect will have negative width or height.
Source
Rect get tallMiddleRect {
final double leftRadius = math.max(blRadiusX, tlRadiusX);
final double rightRadius = math.max(trRadiusX, brRadiusX);
return new Rect.fromLTRB(
left + leftRadius,
top,
right - rightRadius,
bottom
);
}