Some times on css z-index work on the other hand it doesnt work

Question

css for popup manu

Answer 1

Your affected elements must have their css positions different to 'relative' too in order this works.

Answer 2

z-index is not simple like seem... You must understand stacking context

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index

Answer 3

Using z-index may affect performances, so sometimes would be better to find a solution that doesn't use this property.

https://www.google.it/amp/blog.twonegatives.com/post/19178820235/why-z-index-is-probably-not-what-you-want-to-use/amp