Uncaught ReferenceError: mfTempOpenSection is not defined
at HTMLHeadingElement.onclick https://m.mediawiki.org/wiki/MediaWiki:106:1497
Sections cannot be opened by users on very slow mobile connections on the main page.
This happens during the first few seconds of the page visit for all users (depending on connection and device speed).
And it happens all the time for users of a Grade C browser, and for users where the JS request failed or timed out (e.g. intermittent connectivity or a after bad deploy).
This appears to occur on the home page e.g. https://m.mediawiki.org/wiki/MediaWiki
Interim toggling supported is added via global function mfTempOpenSection inside https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/includes/MobileFrontendSkinHooks.php#L14
for some reason this is not added on the main page
There is no reason not to add the code on the main page. Some main pages will not use it but that's not a big problem.
Fix is therefore straightforward.