Nested fieldsets and collapsible in Druapl 8

We’ve recently moved our site from Drupal 7 to 8 and I am in the process of upgrading a few modules.

I have a question relating to the replacement of fieldsets with details. 

So previously in Drupal 7 I could.

$form[‘fieldset1’]= array(
    ‘#type’ => ‘fieldset’,
    #collapsible’ => FALSE,
    ‘#collapsed’ => FALSE
);

$form[‘fieldset1’][‘fieldset2’]= array(
    ‘#type’ => ‘fieldset’,
    #collapsible’ => TRUE,
    ‘#collapsed’ => FALSE
);

Which would allow me to nest a fieldset inside a previous fieldset and also define if it is collapsible or not

The equivalent in Drupal 8 would be.

$form[‘details1’]= array(
    ‘#type’ => ‘details’,
    ‘#open’ => TRUE,
);

$form[‘details1’][‘details2’]= array(
    ‘#type’ => ‘details’,
    ‘#open’ => TRUE,
);

But 1) they are not nested and 2) I now can’t make them collapsible.

From what I have read I believe this to be correct, could somebody just confirm that for me, please? As I can’t find any specific information relating to this.

Drupal version: 


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