XmManager
Motif's abstract base class for widgets that hold other widgets, and the thing that quietly makes gadgets work.
XmManager inherits from Xt's Constraint class, so each child can carry layout resources for its parent to read. The layout itself is left to the subclasses — XmBulletinBoard, XmForm, XmRowColumn, XmFrame, XmPanedWindow, XmScrolledWindow, XmDrawingArea and others — while XmManager supplies the shared parts: shadows and colours matching XmPrimitive, tab traversal between children, and XmNinitialFocus to pick which child gets focus first.
Its most important job is invisible. A gadget has no X window of its own and so cannot receive events, so the manager catches clicks and keystrokes in its own window, works out whose rectangle they landed in, and forwards them. It also draws the gadgets' focus highlights. The entire saving behind windowless menu items rests on every Motif container having this little switchboard built in.