performLayout method
Implementation
@override
void performLayout(BoxConstraints constraints) {
final innerW = (constraints.maxWidth - 2 * borderSize).clamp(0, 0x3FFFFFFF);
final innerH = (constraints.maxHeight - 2 * borderSize).clamp(0, 0x3FFFFFFF);
child?.layout(BoxConstraints.tight(Size(innerW, innerH)));
child?.setOffset(borderSize, borderSize);
setSize(constraints.maxWidth, constraints.maxHeight);
}