ctrlutil.go
func FindFirstControl(parent Control, fn func(Control) bool) Control {
	linear := getLinearControlList(parent, fn)
	if len(linear) == 0 {
		return nil
	}

	return linear[0]
}
func FindFirstControl(parent Control, fn func(Control) bool) Control {
	linear := getLinearControlList(parent, fn)
	if len(linear) == 0 {
		return nil
	}

	return linear[0]
}
// FindFirstControl returns the first child for that fn returns true.
// The function is used to find active or tab-stop control