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Use .z-index-{n2|n1|0|1|2|master} class to change z-position of an element.
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$zindex-dropdown: 1000;
$zindex-sticky: 1020;
$zindex-fixed: 1030;
$zindex-modal-backdrop: 1040;
$zindex-offcanvas: 1050;
$zindex-modal: 1060;
$zindex-popover: 1070;
$zindex-tooltip: 1080;