full site editing – Position tab missing in the Block Theme Editor

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full site editing – Position tab missing in the Block Theme Editor

Question

Hello,
Using WordPress 6.2.2 with a custom block theme created from scratch, and I’m trying to make my header sticky. And what I thought would be an easy task then became a challenge.
I got to find: This official WordPress resource explaining how to do that, but they have a position tab I haven’t seen. Do I have to activate something in my theme.json? As it not work in CSS too, I think some classes got to cancel the sticky thing.
Thank you very much for helping me!

Images are what they have, VS what I have: