GtkNotebookset_tab_detachable

Sets whether the tab can be detached from notebook to another notebook or widget.

Note that two notebooks must share a common group identifier (see gtk_notebook_set_group_name()) to allow automatic tabs interchange between them.

If you want a widget to interact with a notebook through DnD (i.e.: accept dragged tabs from it) it must be set as a drop destination by adding to it a GtkDropTarget controller that accepts the GType GTK_TYPE_NOTEBOOK_PAGE. The :value of said drop target will be preloaded with a GtkNotebookPage object that corresponds to the dropped tab, so you can process the value via ::accept or ::drop signals.

Note that you should use gtk_notebook_detach_tab() instead of gtk_notebook_remove_page() if you want to remove the tab from the source notebook as part of accepting a drop. Otherwise, the source notebook will think that the dragged tab was removed from underneath the ongoing drag operation, and will initiate a drag cancel animation.

static void
on_drag_data_received (GtkWidget        *widget,
                       GdkDrop          *drop,
                       GtkSelectionData *data,
                       guint             time,
                       gpointer          user_data)
{
  GtkDrag *drag;
  GtkWidget *notebook;
  GtkWidget **child;

  drag = gtk_drop_get_drag (drop);
  notebook = g_object_get_data (drag, "gtk-notebook-drag-origin");
  child = (void*) gtk_selection_data_get_data (data);

  // process_widget (*child);

  gtk_notebook_detach_tab (GTK_NOTEBOOK (notebook), *child);
}

If you want a notebook to accept drags from other widgets, you will have to set your own DnD code to do it.