Managing z-index layers is the worst, so we use this document to manually track what layers each piece of our application lives on.
Inline, absolutely positioned content
The option list that is presented by dropdowns.
The site navigation gets its own layer. It should appear above all website content on mobile devices, but it should always be the lowest in the stack of elements that overtake the rest of the website.
Elements that should take over the entire website, such as alerts and dialogs