During research on T263832: Major performance regression on mobile site associated with 1.36.0-wmf.10 , it's appeared that adding collapsible to section was dropped during refactoring but tests show that result is correct.

So, more tests need to be added

The tests should assert that before any javascript has executed that sections appear hidden. I.e. a browser test that executes immediately on page load.

I'm not sure how we'd do this, as it feels like a browser test but requires execution prior to any Javascript loading. It's not clear if webdriver supports such a thing. Maybe testing team have some recommendations here.