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This is strictly by convention.  It makes it possible for an arbitrary
widget to know that the visual it uses can be found by looking for the
shell widget that is its ancestor and obtaining the visual of that
shell.

A widget can have its own visual resource.  If it does, it must have
its own realize method to use the visual when it calls
XCreateWindow().  You should also make this a resource that can be
obtained with XtGetValues() so other widgets can find it.  A
reasonable value is probably XtNvisual.