Auto close (hide) custom metabox / set default state

Hook into postbox_classes. postbox_classes is the function which will output the classes for the metabox.

apply_filters( "postbox_classes_{$page}_{$id}", $classes )

Your code could look like this:

add_action( 'add_meta_boxes', 'add_my_metabox' );

function add_my_metabox() {
  $id       = 'my-metabox';
  $title="My Metabox";
  $callback = 'my_metabox_content';
  $page="post";

  add_meta_box( $id, $title, $callback, $page );

  add_filter( "postbox_classes_{$page}_{$id}", 'minify_my_metabox' );
}

function my_metabox_content() { ... }


/*
 * $classes is an array
 */
function minify_my_metabox( $classes ) {
  if ( isset( $_POST['my_condition'] ) && 'my_condition' == $_POST['my_condition'] ) {
    array_push( $classes, 'closed' );
  }

  return $classes;
}