z-index
z-index is the CSS property that helps control layout by providing a third axis to arrange content.
Several Material UI components utilize z-index, employing a default z-index scale
that has been designed to properly layer drawers, modals, snackbars, tooltips, and more.
The z-index values start at an arbitrary number, high and specific enough to ideally avoid conflicts:
- mobile stepper: 1000
 - fab: 1050
 - speed dial: 1050
 - app bar: 1100
 - drawer: 1200
 - modal: 1300
 - snackbar: 1400
 - tooltip: 1500
 
These values can always be customized.
You will find them in the theme under the zIndex key of the theme.
Customization of individual values is discouraged; should you change one, you likely need to change them all.