Saves a draft or manually autosaves for the purpose of showing a post preview.

function post_preview() {

	$post_ID     = (int) $_POST['post_ID'];
	$_POST['ID'] = $post_ID;

	$post = get_post( $post_ID );

	if ( ! $post ) {
		wp_die( __( 'Sorry, you are not allowed to edit this post.' ) );
	}

	if ( ! current_user_can( 'edit_post', $post->ID ) ) {
		wp_die( __( 'Sorry, you are not allowed to edit this post.' ) );
	}

	$is_autosave = false;

	if ( ! wp_check_post_lock( $post->ID ) && get_current_user_id() == $post->post_author
		&& ( 'draft' === $post->post_status || 'auto-draft' === $post->post_status )
	) {
		$saved_post_id = edit_post();
	} else {
		$is_autosave = true;

		if ( isset( $_POST['post_status'] ) && 'auto-draft' === $_POST['post_status'] ) {
			$_POST['post_status'] = 'draft';
		}

		$saved_post_id = wp_create_post_autosave( $post->ID );
	}

	if ( is_wp_error( $saved_post_id ) ) {
		wp_die( $saved_post_id->get_error_message() );
	}

	$query_args = array();

	if ( $is_autosave && $saved_post_id ) {
		$query_args['preview_id']    = $post->ID;
		$query_args['preview_nonce'] = wp_create_nonce( 'post_preview_' . $post->ID );

		if ( isset( $_POST['post_format'] ) ) {
			$query_args['post_format'] = empty( $_POST['post_format'] ) ? 'standard' : sanitize_key( $_POST['post_format'] );
		}

		if ( isset( $_POST['_thumbnail_id'] ) ) {
			$query_args['_thumbnail_id'] = ( (int) $_POST['_thumbnail_id']