Bootstrap 4 components z-index values

Several Bootstrap components utilize z-index, the CSS property that helps control layout by providing a third axis to arrange content. Bootstrap 4 utilize a default z-index scale that’s been designed to properly layer navigation, tooltips and popovers, modals, and more.

$zindex-dropdown:          1000 !default;
$zindex-sticky:            1020 !default;
$zindex-fixed:             1030 !default;
$zindex-modal-backdrop:    1040 !default;
$zindex-modal:             1050 !default;
$zindex-popover:           1060 !default;
$zindex-tooltip:           1070 !default;