How do I sort the results of a recursive query in an expanded tree-like fashion?

Let's assume you have a nodes tables like this:

CREATE TABLE nodes (    node serial PRIMARY KEY,    parent integer NULL REFERENCES nodes(node),    ts timestamp NOT NULL DEFAULT now());

It represents a standard node-like tree structure with root nodes at the top and several child nodes dangling from root nodes or other child nodes.

Let us insert a couple of example values:

INSERT INTO nodes (parent) VALUES  (NULL), (NULL), (NULL), (NULL), (1), (1), (1), (1), (6), (1), (6), (9), (6), (6), (3), (3), (3), (15);

Now I want to retrieve the first 10 root nodes and all their children up to a depth of 4:

WITH RECURSIVE node_rec AS (    (SELECT 1 AS depth, * FROM nodes WHERE parent IS NULL LIMIT 10)    UNION ALL    SELECT depth + 1, n.*    FROM nodes AS n JOIN node_rec ON (n.parent = node_rec.node)    WHERE depth < 4)SELECT * FROM node_rec;

This works great and gives me the following result:

 depth | node | parent -------+------+--------     1 |  1   |     1 |  2   |     1 |  3   |     1 |  4   |     2 |  5   |  1     2 |  6   |  1     2 |  7   |  1     2 |  8   |  1     2 | 10   |  1     2 | 15   |  3     2 | 16   |  3     2 | 17   |  3     3 |  9   |  6     3 | 11   |  6     3 | 13   |  6     3 | 14   |  6     3 | 18   | 15     4 | 12   |  9

As you might have noticed, there's no ORDER BY clause, so the order is not defined. The order you see here is from root nodes to deeper nodes.

How would I order the results as they would appear in an expanded tree view, as you can see from the example picture below?

I basically want the child nodes to be placed right after their corresponding parent node. If two or more child nodes have the same parent node, I want them to be sorted by their timestamp. Based on the example above, here's the desired output order that I'm trying to achieve:

 depth | node | parent | ts-------+------+--------+---------     1 |  1   |        | 2014-01-01 00:00:00     2 |  5   |     1  | 2014-01-01 00:10:00     2 |  6   |     1  | 2014-01-01 00:20:00     3 |  9   |     6  | 2014-01-01 00:25:00     4 |  12  |     9  | 2014-01-01 00:27:00     3 |  11  |     6  | 2014-01-01 00:26:00     3 |  13  |     6  | 2014-01-01 00:30:00     3 |  14  |     6  | 2014-01-01 00:36:00     2 |  7   |     1  | 2014-01-01 00:21:00     2 |  8   |     1  | 2014-01-01 00:22:00     2 |  10  |     1  | 2014-01-01 00:23:00     1 |  2   |        | 2014-01-01 00:08:00     1 |  3   |        | 2014-01-01 00:09:00     2 |  15  |     3  | 2014-01-01 10:00:00     3 |  18  |     15 | 2014-01-01 11:05:00     2 |  16  |     3  | 2014-01-01 11:00:00     2 |  17  |     3  | 2014-01-01 12:00:00     1 |  4   |        | 2014-01-01 00:10:00