For some reason I cannot get the theme_fieldset function to make me a collapsible or collapsed fieldset.
$page['xyz'] = theme('fieldset', array('element' => array('#title' => 'XYZ', '#value' => $xyz, '#collapsed' => TRUE, '#collapsible' => TRUE)));
should as far as I can see from the docs give me a collapsible fieldset starting out collapsed.
It gives me a fieldset, but neither collapsed nor collapsible.
I can't find any examples online, and honestly; looking in the code I can't see where the theme function actually checks for these parameters and creates the fieldset appropriately. Looking through my D7 installation, I find one reference to theme_fieldset or theme('fieldset' ...), apart from in my own code, and that's in form.inc where the function is declared.
Am I theming the fieldset the completely wrong way?