Once created a widget will usually want to be managed. XtManageChild() is a function that achieves this.
When this happens all aspects of its sizing are taken control of by the parent.
We will look at this in more detail later.
Let us leave this topic by noting that we can actually create and manage a widget in one function call XtVaCreateManagedWidget(). This function can be used to create any widget. We will meet this later in the course.