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After it is realized, one doesn't resize a Shell widget.  The proper
thing is to resize the currently managed child of the Shell widget
using XtSetValues().  The geometry change is then propagated to the
Shell which asks the window manager which may or may not allow the
request.  However, the Shell must have the resource
XtNallowShellResize set to True otherwise it will not even ask the
window manager to grant the request and the Shell will not resize.

To change the position of a Shell, use XtSetValues() on the Shell, not
the child, and within the limits of the window manager it should be granted.