GtkContainer checklist
Because there's no cramming feature in GtkFlowBox I wrote my own container, and to do so I had to look through different documentation. For instance it kept complaining that there's no window to realize with no hint as how to proceed, and only from gtkmm documentation I read that I should call gtk_widget_set_has_window to tell GTK that there should be no window. I thought it would be nice to have a TODO-style checklist on how to implement containers.