feat: add underline variant to tabs

Author: mrholek

docs/content/components/navs-tabs.md

@@ -168,6 +168,27 @@ Take that same HTML, but use `.nav-pills` instead:
 </ul>
 {{< /example >}}
 
+### Underline
+
+Take that same HTML, but use `.nav-underline` instead:
+
+{{< example >}}
+<ul class="nav nav-underline">
+  <li class="nav-item">
+    <a class="nav-link active" aria-current="page" href="#">Active</a>
+  </li>
+  <li class="nav-item">
+    <a class="nav-link" href="#">Link</a>
+  </li>
+  <li class="nav-item">
+    <a class="nav-link" href="#">Link</a>
+  </li>
+  <li class="nav-item">
+    <a class="nav-link disabled" aria-disabled="true">Disabled</a>
+  </li>
+</ul>
+{{< /example >}}
+
 ### Fill and justify
 
 Force your `.nav`'s contents to extend the full available width one of two modifier classes. To proportionately fill all available space with your `.nav-item`s, use `.nav-fill`. Notice that all horizontal space is occupied, but not every nav item has the same width.

scss/_nav.scss

@@ -16,7 +16,7 @@
 
   display: flex;
   flex-wrap: wrap;
-  @include ltr-rtl("padding-left", 0);
+  padding-left: 0;
   margin-bottom: 0;
   list-style: none;
 }
@@ -44,7 +44,8 @@
   }
 
   // Disabled state lightens text
-  &.disabled {
+  &.disabled,
+  &:disabled {
     color: var(--#{$prefix}nav-link-disabled-color);
     pointer-events: none;
     cursor: default;
@@ -70,7 +71,6 @@
 
   .nav-link {
     margin-bottom: calc(-1 * var(--#{$prefix}nav-tabs-border-width)); // stylelint-disable-line function-disallowed-list
-    background: none;
     border: var(--#{$prefix}nav-tabs-border-width) solid transparent;
     @include border-top-radius(var(--#{$prefix}nav-tabs-border-radius));
 
@@ -80,13 +80,6 @@
       isolation: isolate;
       border-color: var(--#{$prefix}nav-tabs-link-hover-border-color);
     }
-
-    &.disabled,
-    &:disabled {
-      color: var(--#{$prefix}nav-link-disabled-color);
-      background-color: transparent;
-      border-color: transparent;
-    }
   }
 
   .nav-link.active,
@@ -117,15 +110,7 @@
   // scss-docs-end nav-pills-css-vars
 
   .nav-link {
-    background: none;
-    border: 0;
     @include border-radius(var(--#{$prefix}nav-pills-border-radius));
-
-    &:disabled {
-      color: var(--#{$prefix}nav-link-disabled-color);
-      background-color: transparent;
-      border-color: transparent;
-    }
   }
 
   .nav-link.active,
@@ -136,6 +121,39 @@
 }
 
 
+//
+// Underline
+//
+
+.nav-underline {
+  // scss-docs-start nav-underline-css-vars
+  --#{$prefix}nav-underline-gap: #{$nav-underline-gap};
+  --#{$prefix}nav-underline-border-width: #{$nav-underline-border-width};
+  --#{$prefix}nav-underline-link-active-color: #{$nav-underline-link-active-color};
+  // scss-docs-end nav-underline-css-vars
+
+  gap: var(--#{$prefix}nav-underline-gap);
+
+  .nav-link {
+    padding-right: 0;
+    padding-left: 0;
+    border-bottom: var(--#{$prefix}nav-underline-border-width) solid transparent;
+
+    &:hover,
+    &:focus {
+      border-bottom-color: currentcolor;
+    }
+  }
+
+  .nav-link.active,
+  .show > .nav-link {
+    font-weight: $font-weight-bold;
+    color: var(--#{$prefix}nav-underline-link-active-color);
+    border-bottom-color: currentcolor;
+  }
+}
+
+
 //
 // Justified variants
 //

scss/_variables.scss

@@ -1232,6 +1232,10 @@ $nav-tabs-link-active-border-color: var(--#{$prefix}border-color) var(--#{$prefi
 $nav-pills-border-radius:           var(--#{$prefix}border-radius) !default;
 $nav-pills-link-active-color:       $component-active-color !default;
 $nav-pills-link-active-bg:          $component-active-bg !default;
+
+$nav-underline-gap:                 1rem !default;
+$nav-underline-border-width:        .125rem !default;
+$nav-underline-link-active-color:   var(--#{$prefix}emphasis-color) !default;
 // scss-docs-end nav-variables