This might be fixed by Arlo's patch this week (Html headings aren't section wrapped (1244822)), since it appears this is styling around raw <h2> tags generated by
https://fr.wikipedia.org/w/index.php?title=Template:Section%20d%C3%A9roulante%20d%C3%A9but&action=edit

In T421859#11773860, @cscott wrote:

This might be fixed by Arlo's patch this week

Yes, post-deploy of T417530 and purge, the major rendering difference is fixed

There is a minor issue though that the mw:DisplaySpace span in the .cdx-button gets a gap: 6px applied to it, making a the space between the text and question mark unreasonable. It can probably fixed by added gap: 0px; to the inline style set in {{bouton cliquable}} but maybe there's a better option

In T421859#11784426, @ABreault-WMF wrote:

There is a minor issue though that the mw:DisplaySpace span in the .cdx-button gets a gap: 6px applied to it, making the space between the text and question mark unreasonable. It can probably be fixed by adding gap: 0px; to the inline style set in {{bouton cliquable}} but maybe there's a better option

The gap on .cdx-button was added in T356103: Natural space between icon and button text is lost in MinervaNeue. I added the inline style you suggested to the template.