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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.