Happy to review a patch from a volunteer if someone wants to have a go at this!

The fault tree of this problem is
1: skins.minerva.toggling/init.js tries to upgrade window.mfTempOpenSection to a 'mobile.toggle/Toggler'
2: it filters headers as follows: $container.find( '> h1,> h2,> h3,> h4,> h5,> h6' ). This is correct for Toggler, but not for this situation, since mfTempOpenSection is added to EVERY header initially. It's original addition does not match this filter.
3: Because of this mismatch in the 'upgrade', the onclick handler is not removed from this 'ineligible' header.
4: At the same time the Toggler initialisation will fail for this header
5: After the skipped onclick removal, it also does: delete window.mfTempOpenSection; to remove this function from the global context
6: That however never works, since delete doesn't operate on non-configurable properties of an object, which this is. And I have no idea why you would do this to begin with, since the global scope is already polluted, cleaning up doesn't really seem useful
7: Thus when clicking the 'non-eligible'-header (because in an infobox), you hit the window.mfTempOpenSection function.
8: The clickhandler function tries to find the section var block=document.getElementById("mf-section-"+id);, but the section was never added, since this is an 'ineligible' header, so the block is not in the HTML
9: The block variable is directly used, without checking if it was actually found.
10: JS error thrown.

Suggested solution:
Change MobileFormatter::prepareHeadings into MobileFormatter::prepareHeading, and prep only those headings for which a section has been created as well (See MobileFormatter::makeSections)

Change 353293 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/extensions/MobileFrontend@master] Only wrap top level headings

https://gerrit.wikimedia.org/r/353293

Change 353293 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] Only wrap top level headings

https://gerrit.wikimedia.org/r/353293

Tested against a few pages on beta using different combinations of devices and browsers using browserstack. Everything looks good.

@phuedx If you've tested the error case, I think this can move to signoff.

Per T152055#3262538 and T152055#3265972.

This is a technical task and @ABorbaWMF has tested and the beta cluster example is now working as expected. I've added a note to test the Wikivoyage example in the calendar.