Steps to replicate the issue (include links if applicable):

• Throttle your connection to 3G
• Load https://en.wikipedia.org/wiki/Paris?useparsoid=1
• Click X to abandon loading as soon as the headings render but before the arrow icon renders
• Click section heading

What happens?:
It doesn't open. The page is unreadable

What should have happened instead?:
The sections should open.

The old legacy parser achieves this by a function MakeSectionsTransform::interimTogglingSupport.
For Parsoid add something like the following inline script:

window.mfTempClickHandler = document.addEventListener('click', ( ev ) => {
	const heading = ev.target.closest( '.mw-heading');
    heading.nextSibling.classList.add('mf-collapsible-content')
});

... and remove it inside src/mobile.init/toggling.js like so:

			document.removeEventListener( 'click', window.mfTempClickHandler );

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):