refactor: update z-indexes

Author: mrholek

scss/_header.scss

@@ -63,7 +63,7 @@
   &.header-sticky {
     position: sticky;
     top: 0;
-    z-index: $zindex-fixed - 1;
+    z-index: $zindex-sticky;
   }
 }
 

scss/_variables.scss

@@ -1195,10 +1195,10 @@ $form-validation-states: (
 $zindex-dropdown:                   1000 !default;
 $zindex-sticky:                     1020 !default;
 $zindex-fixed:                      1030 !default;
+$zindex-sidebar-backdrop:           1034 !default;
+$zindex-sidebar:                    1035 !default;
 $zindex-offcanvas-backdrop:         1040 !default;
-$zindex-sidebar-backdrop:           1040 !default;
 $zindex-offcanvas:                  1045 !default;
-$zindex-sidebar-overlaid:           1045 !default;
 $zindex-modal-backdrop:             1050 !default;
 $zindex-modal:                      1055 !default;
 $zindex-popover:                    1070 !default;

scss/sidebar/_sidebar-narrow.scss

@@ -9,14 +9,12 @@
   }
 
   @include media-breakpoint-up($mobile-breakpoint) {
-    z-index: $zindex-fixed + 1;
     flex: 0 0 var(--#{$prefix}sidebar-narrow-width);
     width: var(--#{$prefix}sidebar-narrow-width);
     padding-bottom: var(--#{$prefix}sidebar-toggler-height);
     overflow: visible;
 
     &.sidebar-fixed {
-      z-index: $zindex-fixed + 1;
       width: var(--#{$prefix}sidebar-narrow-width);
     }
 
@@ -77,7 +75,6 @@
 
 .sidebar-narrow-unfoldable {
   position: fixed;
-  z-index: $zindex-fixed + 1;
 
   &:not(.sidebar-end) ~ * {
     --#{$prefix}sidebar-occupy-start: #{$sidebar-narrow-width};

scss/sidebar/_sidebar.scss

@@ -2,14 +2,14 @@
 
 .sidebar {
   // scss-docs-start sidebar-css-vars
+  --#{$prefix}sidebar-zindex: #{$zindex-sidebar};
   --#{$prefix}sidebar-width: #{$sidebar-width};
   --#{$prefix}sidebar-bg: #{$sidebar-bg};
   --#{$prefix}sidebar-padding-x: #{$sidebar-padding-x};
   --#{$prefix}sidebar-padding-y: #{$sidebar-padding-y};
   --#{$prefix}sidebar-color: #{$sidebar-color};
   --#{$prefix}sidebar-brand-color: #{$sidebar-brand-color};
   --#{$prefix}sidebar-brand-bg: #{$sidebar-brand-bg};
-  --#{$prefix}sidebar-overlaid-zindex: #{$zindex-sidebar-overlaid};
   // scss-docs-end sidebar-css-vars
 
   position: relative;
@@ -25,6 +25,12 @@
   box-shadow: none;
   @include transition($sidebar-transition);
 
+  &.sidebar-fixed,
+  &.sidebar-narrow,
+  &.sidebar-narrow-unfoldable {
+    z-index: var(--#{$prefix}sidebar-zindex);
+  }
+
   &:not(.sidebar-end){
     @include ltr-rtl("margin-left", 0);
     ~ * {
@@ -103,7 +109,6 @@
     position: fixed;
     top: 0;
     bottom: 0;
-    z-index: var(--#{$prefix}sidebar-overlaid-zindex);
 
     &:not(.hide) {
       box-shadow: $box-shadow;
@@ -130,7 +135,7 @@
     position: fixed;
     top: 0;
     bottom: 0;
-    z-index: var(--#{$prefix}sidebar-overlaid-zindex);
+    z-index: var(--#{$prefix}sidebar-zindex);
 
     &:not(.sidebar-end) {
       @include ltr-rtl("left", 0);