fieldset not collapsible

Problem/Motivation

I'd like to be able to collapse the 'permissions' fieldset in the taxonomy term.
I see that the code has in fact :
'#collapsible' => TRUE,
'#collapsed' => TRUE,
But this does not seem to work.

Proposed resolution

Using 'details' instead of 'fieldset' should resolve the problem.

Remaining tasks

Add a conf to collapse or not by default.

User interface changes

The UI permission will be collapsed by default.

API changes