Was created as a result of the copy/paste mechanism, at this point you can count on glade to follow up with properties and children on its own.
Was created during the load process.
Never used.
Was created as a replacement for another project object; this only happens when the user is changing a property that is marked by the type system as G_PARAM_SPEC_CONSTRUCT_ONLY.
Was created at the user's request (this is a good time to set any properties or add children to the project; like GtkFrame's label for example).
These are the reasons your #GladePostCreateFunc can be called.