@Ivan-r: Thanks for reporting this. For future reference, please use the bug report form (linked from the top of the task creation page) to create a bug report, and fill in the sections in the template. Thanks.
Which MediaWiki version, which exact version of the CollapsibleVector extension?

Attaching the screenshot here so it does not get lost on third-party websites, see https://www.mediawiki.org/wiki/Phabricator/Help#Uploading_file_attachments :

@Aklapper, MediaWiki — 1.38.2; CollapsibleVector — 0.1.11 (238abab)

MediaWiki — 1.38.2

That's an outdated insecure version. Please upgrade for your own safety.

I'm experiencing this on MediaWiki 1.39.2 with the latest REL1_39 version of the extension (0.1.11). The master version only has some dependencies that are different, the extension code itself is identical.

The issue is in this section of code, but I'm not sure what can be done to fix it:

				$( this ).find( '.vector-menu-heading' ).wrapInner(
					$( '<a>' )
						.attr( {
							href: '#',
							'aria-haspopup': 'true',
							'aria-controls': id + '-list',
							role: 'button'
						} )
						.on( 'click', false )
				);

Change #1021554 had a related patch set uploaded (by Fomafix; author: Fomafix):

[mediawiki/extensions/CollapsibleVector@master] Use $( … ) instead of mw.hook( 'wikipage.content' ).add( … )

https://gerrit.wikimedia.org/r/1021554