`Category::sort()` might not be needed.

[live627]

The original commit fcb140d tried to fix this by offloading sorting to PHP. However, board_order is reliably updated every time a board is changed:

SMF/Sources/Board.php

Lines 1554 to 1585 in fc87e75

	/**
	* Put all boards in the right order and sorts the records of the boards table.
	* Used by Board::modify(), Board::delete(), Category::modify(), and Category::delete()
	*/
	public static function reorder(): void
	{
	Category::getTree();
	// Set the board order for each category.
	$board_order = 0;
	foreach (Category::$loaded as $cat_id => $dummy) {
	foreach (Category::$boardList[$cat_id] as $board_id) {
	if (self::$loaded[$board_id]->order != ++$board_order) {
	Db::$db->query(
	'UPDATE {db_prefix}boards
	SET board_order = {int:new_order}
	WHERE id_board = {int:selected_board}',
	[
	'new_order' => $board_order,
	'selected_board' => $board_id,
	],
	);
	self::$loaded[$board_id]->order = $board_order;
	}
	}
	}
	// Empty the board order cache
	CacheApi::put('board_order', null, -3600);
	}

Moreover, my code here asserts that the two methods do indeed produce the same order.

	public function listBoards(int $id_board): array
	{
		$request = Db::$db->query(
			'
			SELECT id_board, b.name, child_level, c.name AS cat_name, id_cat
			FROM {db_prefix}boards AS b
				LEFT JOIN {db_prefix}categories AS c USING (id_cat)
			ORDER BY board_order',
		);
		$boards = [];

		while ($row = Db::$db->fetch_assoc($request)) {
			if (!isset($boards[$row['id_cat']])) {
				$boards[$row['id_cat']] = [
					'name' => strip_tags($row['cat_name']),
					'boards' => [],
				];
			}

			$boards[$row['id_cat']]['boards'][$row['id_board']] = [
				'name' => strip_tags($row['name']),
				'child_level' => $row['child_level'],
				'selected' => $row['id_board'] == $id_board,
			];
		}
		Db::$db->free_result($request);

		$a = $boards;

		// Find the boards/categories they can see.
		$boardListOptions = [
			'not_redirection' => true,
			'selected_board' => $id_board,
		];

		if (!empty(Config::$modSettings['recycle_enable']) && !empty(Config::$modSettings['recycle_board'])) {
			$boardListOptions['excluded_boards'] = [(int) Config::$modSettings['recycle_board']];
		}

		$boards = \SMF\Actions\MessageIndex::getBoardList($boardListOptions);

		// Make the board safe for display.
		foreach ($boards as $id_cat => $cat) {
			$boards[$id_cat]['name'] = Utils::htmlspecialcharsDecode(strip_tags($cat['name']));

			foreach ($cat['boards'] as $id_board => $board) {
				$boards[$id_cat]['boards'][$id_board]['name'] = Utils::htmlspecialcharsDecode(strip_tags($board['name']));
			}
		}

		$b = $boards;

		var_dump(array_keys($a) === array_keys($b)); // categories

		foreach ($a as $k => $cat) {
			if (array_keys($cat['boards']) !== array_keys($b[$k]['boards'])) {
				echo "Mismatch in category $k\n";
			}
		}

		return $boards;
	}