centerOf method
Calculates the center of a RenderBox in global screen coordinates.
Uses localToGlobal(Offset.zero) to convert the top-left corner to global coordinates, then adds half the widget's size.
Implementation
Offset centerOf(RenderBox renderBox) {
final origin = renderBox.localToGlobal(Offset.zero);
return origin + Offset(renderBox.size.width / 2, renderBox.size.height / 2);
}