XmBulletinBoard
The simplest of Motif's manager widgets, placing children exactly where their x and y say, and, as the base class of XmForm and every dialog box, the home of all of Motif's dialog behaviour.
The name is literal: a bulletin board where each child is pinned at its own coordinates, XmNx and XmNy, with the widget making no attempt to arrange them. That makes it useless for windows that must survive resizing, but ideal as the backing for interface builders like UIL and Builder Xcessory, where the designer has dragged every button to a pixel position. The XmNresizePolicy resource decides whether the board grows to fit its children or lets them be clipped.
The interesting part is the class hierarchy. XmForm is a bulletin board with attachments bolted on, and XmMessageBox, XmSelectionBox, XmFileSelectionBox and XmCommand all inherit from it. That is why the shared behaviour of dialogs lives here: XmNdefaultButton, which Return presses; XmNcancelButton, which Escape picks; XmNdialogStyle, which makes the box modal for the application or for the whole system; XmNdialogTitle for the shell's title bar; and XmNautoUnmanage, which closes the box once a button has been pressed. XmCreateBulletinBoardDialog creates one directly inside a DialogShell. Anyone who has wondered why Escape dismisses every Motif dialog has their answer here.