By sun on
Change record status: 
Published (View all published change records)
Project: 
Drupal core
Introduced in branch: 
8.x
Issue links: 
https://www.drupal.org/node/1892182
Description: 

The #collapsed property of #type 'details' has been renamed to #open and its value/meaning is negated now.

The default value of #open is FALSE, because HTML5 <details> elements are closed by default, and because <details>, as the element name implies, are supposed to contain details only.

Before

// Advanced settings, closed by default.
$form['advanced'] = array(
  '#type' => 'details',
  '#collapsed' => TRUE,
);
// Grouped controls, not advanced (and not to be skipped), open by default.
$form['some_group'] = array(
  '#type' => 'details',
  '#collapsed' => FALSE,
);

After

// Advanced settings, closed by default.
$form['advanced'] = array(
  '#type' => 'details',
);
// Grouped controls, not advanced (and not to be skipped), open by default.
// Consider to use the new #type 'fieldgroup' instead.
$form['some_group'] = array(
  '#type' => 'details',
  '#open' => TRUE,
);
Impacts: 
Module developers
Themers