Created attachment 331453 [details]
Minimal script to illustrate the problem

It's not possible to inherit from a class that uses a composite template, because gtk_widget_template_init() is called in the subclass's instance init rather than the parent's.

That's not a very clear explanation, so see attached script that illustrates the problem.

I am at a loss as to how to fix this. It seems that the code currently assumes that only one JS _instance_init() function is to be called per object construction and that it should be called from the most derived class's GObject instance init. As per the documentation of gtk_widget_init_template(), "This function must be called in the instance initializer for any class which assigned itself a template using gtk_widget_class_set_template()", which I interpret to mean that init_template() should be called from the superclass's GObject instance init. This might mean that we have to move template initialization to C.