Z-index

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

These higher values ​​start at an arbitrary number, high and specific enough to ideally avoid conflicts. We need a standard set of these across our layered components—tooltips, popovers, navbars, dropdowns, modals—so we can be reasonably consistent in the behaviors. There's no reason we couldn't have used 100+ or 500+.

We don't encourage customization of these individual values; should you change one, you likely need to change them all.

$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;

To handle overlapping borders within components (eg, buttons and inputs in input groups), we use low single digit z-indexvalues ​​of 1, 2, and 3for default, hover, and active states. On hover/focus/active, we bring a particular element to the forefront with a higher z-indexvalue to show their border over the sibling elements.