TextView move overlay failed assertion
gtk_text_view_move_overlay() includes the guard
g_return_if_fail (gtk_widget_get_parent (child) == GTK_WIDGET (text_view));
But the parent is actually
text_view->priv->center_child, set in gtk_text_view_child_add_overlay, so the comparison necessarily fails.
The simplest fix would be to compare the parent with the actual parent. However, that breaks the usual guard structure, so perhaps it would be cleaner to set the
GtkTextView as the parent and leave the guard as it is. I don't know whether that change has ramifications.