gtk_tree_view_enable_model_drag_source doesn't set button mask
gtk_tree_view_enable_model_drag_source calls gtk_drag_source_set but with a mask of 0 instead of the start_button_mask. This makes required to call again gtk_drag_source_set with the right mask when the view is used with a custom Model.
From https://bugzilla.gnome.org/show_bug.cgi?id=756177
Here is a patch and an example treeviewdnd.py showing that DnD does not start if the lines 115-116 are not uncommented