XmForm
Motif's layout widget, where every child is anchored to edges, neighbours or percentage positions rather than given coordinates: the closest Motif came to a layout engine, and the source of the warning about 10,000 iterations.
XmForm is a manager widget, a subclass of XmBulletinBoard, that places its children by constraints. Each child gets four resources, XmNtopAttachment, XmNbottomAttachment, XmNleftAttachment and XmNrightAttachment, each of which can be XmATTACH_FORM (the form's own edge), XmATTACH_WIDGET (next to a sibling), XmATTACH_OPPOSITE_WIDGET, XmATTACH_POSITION (a fraction of XmNfractionBase, 100 by default) or XmATTACH_NONE, plus an offset in pixels. When the window is resized the form recomputes everything, so a button row that should hug the bottom edge or a text field that should fill the width is written as attachments, not as code in a resize callback.
The model was powerful and notorious in equal measure. The constraints are solved iteratively, and contradictory attachments produced the warning Bailed out of edge synchronization after 10,000 iterations, a line generations of Motif programmers learned to recognise. A child with no attachments at all lands wherever XmNx and XmNy point, as in a plain bulletin board. The same idea of anchoring elements to each other resurfaced in iOS Auto Layout and Android's ConstraintLayout twenty years later, with neither acknowledging the ancestry.