Resolved.
There is a problem, very frequent at least in the french Wikipedia, where the first paragraph of the intro "fuses" with the infobox if you use the Visual Editor.
See this example: https://fr.wikipedia.org/w/index.php?diff=147963144 (edit link: https://fr.wikipedia.org/w/index.php?title=Tintin_au_pays_des_Soviets&oldid=147961827&veaction=edit).
Thank you for your work.
Are you concerned about what readers see in the article, or about what the wikitext looks like in the editing box?
I'm concerned about the text acting weirdly in visual editing mode. Try to hover your mouse over the first sentence:
"Tintin au pays des Soviets (titre complet sur la couverture : Les Aventures de Tintin, reporter du « Petit Vingtième », au pays des Soviets) est le premier album de la série de bande dessinée Les Aventures de Tintin, créée par Hergé, dessinateur belge" in this example: https://fr.wikipedia.org/w/index.php?title=Tintin_au_pays_des_Soviets&oldid=147961827&veaction=edit.
The problem, affecting the editing (fluidity or accessibility), should appear clearly.
As you can see here, I dodged the problem by adding a lane between the end of the infobox and the beginning of the article's intro.
Confirmed, it's bugged when I try it. In the link above, in VisualEdit mode, it does not appear possible to directly edit the lead sentence. Trying to click on that sentence is treated as a click on the infobox. As a bizarre workaround, if you edit the infobox you can scroll down to see&edit the lead sentence in the infobox's "Content" field.
It's the dreaded Complex Transclusion dialog box.
Braaark, do you find that this happens consistently with certain infoboxes, but not with others? For example, I can trigger the bug with Modèle:Infobox Cinéma (film) but not with Modèle:Infobox Organisation.
Yes, that happens consistently with certain infoboxes like:
* for books (Infobox Livre), * for albums (Infobox Musique (œuvre)), * for journals (Infobox Revue), * for films (Infobox Cinéma (film)), * for comic book titles (Infobox Comic book) * and for comic strips (Infobox Bande dessinée)
(those are the involved infoboxes I noticed so far, all from the french Wikipedia).
That suggests that the problem is with the infobox templates. Do those particular templates use HTML tags to set absolute positioning (such as <span style="position:absolute;">)? That was a problem once, at a different wiki, and it produced the same symptoms.
I don't know. As I'm not used to HTML language, I signaled the problem here (in french, in the french WP's infobox project page).
@Whatamidoing (WMF), Alsee:: The problem has been solved. It was caused by our template w:fr:Modèle:Infobox/Titre en italique (more informations: , ).