StampScreenOrigin computes and records the screen-space origin on a widget

using the canvas's current transform offset and the widget's local bounds.

 

This should be called by container widgets in their Draw method, after

calling [Canvas.PushTransform] for the container's position, and before

calling Draw on each child widget. The framework also calls this for the

root widget before the first Draw.

 

The screen origin accounts for all accumulated parent transforms including

scroll offsets, making it correct for overlay positioning.

 

Example usage in a container's Draw method:

 

canvas.PushTransform(bounds.Min)

for _, child := range children {

widget.StampScreenOrigin(child, canvas)

child.Draw(ctx, canvas)

}

canvas.PopTransform()

func StampScreenOrigin(child Widget, canvas Canvas) {
	type boundsGetter interface {
		Bounds() geometry.Rect
	}
	type originSetter interface {
		SetScreenOrigin(geometry.Point)
	}

	bg, hasBounds := child.(boundsGetter)
	setter, hasOrigin := child.(originSetter)
	if !hasBounds || !hasOrigin {
		return
	}

	childBounds := bg.Bounds()
	offset := canvas.TransformOffset().Add(canvas.ScreenOriginBase())
	screenOrigin := offset.Add(childBounds.Min)
	if debugStamp {
		fmt.Fprintf(os.Stderr, "[STAMP] %T bounds=%v canvasOffset=%v → screen=%v\n",
			child, childBounds, offset, screenOrigin)
	}
	setter.SetScreenOrigin(screenOrigin)
}