An animation technique to grab the user's attention or just a bug?

Question

css

I constantly observe this chip - a kind of twitching of the entire page before leaving the menu. It's kind of like a fade-out/in effect (or something like that, I'm not very good at animation yet). Here is an example - https://tproger.ru/digest/5-animation-tools-for-web/
So, is this a special effect or just laziness / crooked hands of a developer who uses ready-made libraries for jQuery? (by the way, what kind of library, if it is used here?)

Answer 1

Position: fixed is simply hung on the menu and its height is lost. Its place is occupied by the lower block and therefore jerking occurs.

Answer 2

You should not change the fixed class, but manually set the dimensions at which it should take the values ​​position: fixed;