For context, see T269878#6856049.
The CollapsibleFieldsetLayout is a PHP layout that uses the script jquery.makeCollapsible.js (also used by other components), which works with the class mw-collapsible.
The margins and padding for the CollapsibleFieldsetLayout are defined in mediawiki.htmlform.ooui.styles.less, and are applied to elements with the mw-collapisble class (among others) inside an element with class mw-htmlform-ooui-wrapper.
IPInfo uses CollapsibleFieldsetLayout outside of a form, so the margins and padding are inconsistent:
Rather than re-implement these styles just for IPInfo's use-case, it would be more helpful to create a new CSS stylesheet for CollapsibleFieldsetLayout that applies these styles. We'd also need to add a CSS class that is specific to the CollapsibleFieldsetLayout. This allows the layout to be reused more flexibly.
