How can I implement the appearance of a hidden menu when scrolling to the desired element using CSS?

Question

css

I have never been knowledgeable about this topic in general. Therefore, having studied the issue, I myself came to the conclusion that in my version this is most likely possible by playing with opacity in CSS.
If, in fact, the site is an example
, when scrolling on the header, a menu appears, in other words, I can create it, hide it under a layer in the header, but how to organize a "collision" of the sticky header on the desired element, so that the trigger clicks (opacity as I understand it in my case ) and it appeared? Maybe there is another way?

Answer 1

the javascript event is called scroll , search in the search engine for the tags "scroll menu" "sticky scroll menu"
here is one way to implement it : Stupidly simple sticky-on-scroll menu
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if done in css then it's called position:sticky
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in your example the menu is implemented in js