We not only need to make sure the iterator remains valid, but also
the object pointed to.
Valgrind trace:
Invalid read of size 8
Gtkmm2ext::Pane::forall_vfunc(int, void (*)(_GtkWidget*, void*), void*) (pane.cc:617)
Gtk::Container_Class::forall_vfunc_callback(_GtkContainer*, int, void (*)(_GtkWidget*, void*), void*)
gtk_container_destroy (gtkcontainer.c:1073)
g_closure_invoke (gclosure.c:804)
...
g_object_run_dispose (gobject.c:1084)
Gtkmm2Ext::Pane::on_add() uses a pointer to a std::vector<> element
in the destroy notify callback. If the vector is modified, that pointer
becomes invalid.
Add 2 widgets "A", "B". remove "B", add another one "C".
Now if A is destroyed, notify_child_destroyed(PTR) points to
invalid memory and not to "A".
* enforce minimum size of child widgets
* honor manually set child-minsize in size-requests
* ignore hidden children (eg. VCA)
* clamp divider position (instead of just ignoring out-of-bounds moves)
Gtk::Widget_Class::dispose_vfunc_callback calls hide() which invokes
Pane::handle_child_visibility which calls Pane::reallocate which
tries to get the allocation of the widget being destroyed.
Code to check if we were to close to an edge (for window resizing) blocked all divider setting,
because it would be called with a current widget allocation of 1x1
