Z-index utilities let you set the priority order to display elements on top of each others.
Available classes are (from lowest z-index to higher):
.ecl-u-z-highlight: just above normal content.ecl-u-z-navigation: elements meant to be on top of normal content when browsing the page (sticky header, menu, ...).ecl-u-z-dropdown: elements displayed only in some situatuion, on top of the others (select, sub menu, ...).ecl-u-z-modal: elements on top of all others (full screen gallery, modal dialog, ...).ecl-u-z-max: always on top, no matter what