The SimpleSpinBox widget, introduced in Motif 2.1, is derivative of the SpinBox widget. Unlike the SpinBox widget, it is not meant to be a general purpose manager, and you should not add children directly to the SimpleSpinBox. Instead, the widget packages up the usual requirement which is for a SpinBox with a single TextField child. The SimpleSpinBox creates a TextField on behalf of the programmer automatically.
The normal way of specifying the range of values associated with a SpinBox TextField is through constraint resources. This can still be done, however the SimpleSpinBox provides what is called a mirror, so that resources can be applied directly to the SpinBox to specify the range of values associated with the single built-in TextField child.
For more details of the SimpleSpinBox widget, see XmSimpleSpinBox(3X) in the OSF/Motif Programmer's Reference.
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