Quand je presse Fullscreen l'écran de jeu disparait (mais le son reste, ainsi que le site).

Le long press ne fonctionne pas non plus.
Que ce soit avec Safari, Chrome, Firefox ou Edge...

Seulement, je n'ai pas d'iphone pour tester. C'est beaucoup trop cher, et ça ne fonctionne pas, apparemment.

for some reasons when i click full screen this happens

The reason is simple: Apple.
They do everything they can to prevent games that don't come out of their appstore from working properly.

Once again, the most expensive device can't do the most basic HTML5 stuff. And I don't have an iPhone in order to do Apple engineers' job and debug it...

So, if you happen to have one, please go there https://www.txori.com/dc
Click the play ▶️ button
Then the fullscreen ⬛ button in the top-left corner

and tell me if fullscreen works or not...

Thanks!

All right, I got the answer... 10 years after, full screen API *still* isn't supported on iOS:
https://caniuse.com/fullscreen

So please stop buying this useless piece of crap every year, and then go on complaining to developers that their free games don't work on your expensive brick.