Gradients and shadows (twbs#24429)

* Update the form focus mixin to use a manual `$enable-shadows` check so we can always ensure a focus state for accessibility and consistency

* - Add new `$input-btn-focus-width` and `$input-btn-focus-color` variables.

- Replace separate `$btn-focus-box-shadow` and `$input-focus-box-shadow`
variables with unified `$input-btn-focus-box-shadow` to match our
combined variables approach elsewhere.

* Put new focus width var to use in buttons mixins

* use new button input-box shadow var

* Add a new mixin for quickly adding linear gradient to components when $enable-gradients is set to true

* use correct var

* fix focus shadows in button mixins

* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items

* Generate .bg-gradient- utilities

* add headings to colors page and document bg-gradient utils

* update the button color for active status, check with yiq as it's done for basic state and hover state

Author: mdo

docs/4.0/utilities/colors.md

@@ -6,6 +6,8 @@ group: utilities
 toc: true
 ---
 
+## Color
+
 {% example html %}
 {% for color in site.data.theme-colors %}
 <p class="text-{{ color.name }}{% if color.name == "light" %} bg-dark{% endif %}">.text-{{ color.name }}</p>{% endfor %}
@@ -22,6 +24,8 @@ Contextual text classes also work well on anchors with the provided hover and fo
 <p><a href="#" class="text-white bg-dark">White link</a></p>
 {% endexample %}
 
+## Background color
+
 Similar to the contextual text color classes, easily set the background of an element to any contextual class. Anchor components will darken on hover, just like the text classes. Background utilities **do not set `color`**, so in some cases you'll want to use `.text-*` utilities.
 
 {% example html %}
@@ -30,6 +34,15 @@ Similar to the contextual text color classes, easily set the background of an el
 <div class="p-3 mb-2 bg-white text-dark">.bg-white</div>
 {% endexample %}
 
+## Background gradient
+
+When `$enable-shadows` is set to true, you'll be able to use `.bg-gradient-` utility classes. **By default, `$enable-shadows` is disabled and the example below is intentionally broken.** This is done for easier customization from moment you start using Bootstrap. [Learn about our Sass options]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/theming/#sass-options) to enable these classes and more.
+
+{% example html %}
+{% for color in site.data.theme-colors %}
+<div class="p-3 mb-2 bg-gradient-{{ color.name }} {% if color.name == "light" %}text-dark{% else %}text-white{% endif %}">.bg-gradient-{{ color.name }}</div>{% endfor %}
+{% endexample %}
+
 {% callout info %}
 #### Dealing with specificity
 

scss/_buttons.scss

@@ -22,7 +22,7 @@
   &:focus,
   &.focus {
     outline: 0;
-    box-shadow: $btn-focus-box-shadow;
+    box-shadow: $input-btn-focus-box-shadow;
   }
 
   // Disabled comes first so active can properly restyle
@@ -35,7 +35,7 @@
   &:not([disabled]):not(.disabled):active,
   &:not([disabled]):not(.disabled).active {
     background-image: none;
-    @include box-shadow($btn-focus-box-shadow, $btn-active-box-shadow);
+    @include box-shadow($input-btn-focus-box-shadow, $btn-active-box-shadow);
   }
 }
 

scss/_carousel.scss

@@ -90,9 +90,15 @@
 }
 .carousel-control-prev {
   left: 0;
+  @if $enable-gradients {
+    background: linear-gradient(90deg, rgba(0,0,0,.25), rgba(0,0,0,.001));
+  }
 }
 .carousel-control-next {
   right: 0;
+  @if $enable-gradients {
+    background: linear-gradient(270deg, rgba(0,0,0,.25), rgba(0,0,0,.001));
+  }
 }
 
 // Icons for within

scss/_custom-forms.scss

@@ -22,7 +22,7 @@
 
   &:checked ~ .custom-control-indicator {
     color: $custom-control-indicator-checked-color;
-    background-color: $custom-control-indicator-checked-bg;
+    @include gradient-bg($custom-control-indicator-checked-bg);
     @include box-shadow($custom-control-indicator-checked-box-shadow);
   }
 
@@ -33,7 +33,7 @@
 
   &:active ~ .custom-control-indicator {
     color: $custom-control-indicator-active-color;
-    background-color: $custom-control-indicator-active-bg;
+    @include gradient-bg($custom-control-indicator-active-bg);
     @include box-shadow($custom-control-indicator-active-box-shadow);
   }
 
@@ -244,7 +244,7 @@
     padding: $custom-file-padding-y $custom-file-padding-x;
     line-height: $custom-file-line-height;
     color: $custom-file-button-color;
-    background-color: $custom-file-button-bg;
+    @include gradient-bg($custom-file-button-bg);
     border: $custom-file-border-width solid $custom-file-border-color;
     @include border-radius(0 $custom-file-border-radius $custom-file-border-radius 0);
   }

scss/_dropdown.scss

@@ -67,14 +67,14 @@
   @include hover-focus {
     color: $dropdown-link-hover-color;
     text-decoration: none;
-    background-color: $dropdown-link-hover-bg;
+    @include gradient-bg($dropdown-link-hover-bg);
   }
 
   &.active,
   &:active {
     color: $dropdown-link-active-color;
     text-decoration: none;
-    background-color: $dropdown-link-active-bg;
+    @include gradient-bg($dropdown-link-active-bg);
   }
 
   &.disabled,

scss/_variables.scss

@@ -314,6 +314,10 @@ $input-btn-padding-y:         .375rem !default;
 $input-btn-padding-x:         .75rem !default;
 $input-btn-line-height:       $line-height-base !default;
 
+$input-btn-focus-width:       .2rem !default;
+$input-btn-focus-color:       rgba(theme-color("primary"), .25) !default;
+$input-btn-focus-box-shadow:  0 0 0 $input-btn-focus-width $input-btn-focus-color !default;
+
 $input-btn-padding-y-sm:      .25rem !default;
 $input-btn-padding-x-sm:      .5rem !default;
 $input-btn-line-height-sm:    $line-height-sm !default;
@@ -324,7 +328,6 @@ $input-btn-line-height-lg:    $line-height-lg !default;
 
 $btn-font-weight:             $font-weight-normal !default;
 $btn-box-shadow:              inset 0 1px 0 rgba($white,.15), 0 1px 1px rgba($black,.075) !default;
-$btn-focus-box-shadow:        0 0 0 3px rgba(theme-color("primary"), .25) !default;
 $btn-active-box-shadow:       inset 0 3px 5px rgba($black,.125) !default;
 
 $btn-link-disabled-color:     $gray-600 !default;
@@ -355,7 +358,6 @@ $input-border-radius-sm:                $border-radius-sm !default;
 
 $input-focus-bg:                        $input-bg !default;
 $input-focus-border-color:              lighten(theme-color("primary"), 25%) !default;
-$input-focus-box-shadow:                $input-box-shadow, $btn-focus-box-shadow !default;
 $input-focus-color:                     $input-color !default;
 
 $input-placeholder-color:               $gray-600 !default;
@@ -404,7 +406,7 @@ $custom-control-indicator-checked-color:        $white !default;
 $custom-control-indicator-checked-bg:           theme-color("primary") !default;
 $custom-control-indicator-checked-box-shadow:   none !default;
 
-$custom-control-indicator-focus-box-shadow:     0 0 0 1px $body-bg, 0 0 0 3px theme-color("primary") !default;
+$custom-control-indicator-focus-box-shadow:     0 0 0 1px $body-bg, $input-btn-focus-box-shadow !default;
 
 $custom-control-indicator-active-color:         $white !default;
 $custom-control-indicator-active-bg:            lighten(theme-color("primary"), 35%) !default;

scss/mixins/_alert.scss

@@ -1,6 +1,6 @@
 @mixin alert-variant($background, $border, $color) {
   color: $color;
-  background-color: $background;
+  @include gradient-bg($background);
   border-color: $border;
 
   hr {

scss/mixins/_background-variant.scss

@@ -12,3 +12,9 @@
     }
   }
 }
+
+@mixin bg-gradient-variant($parent, $color) {
+  #{$parent} {
+    background: $color linear-gradient(180deg, mix($body-bg, $color, 15%), $color) repeat-x !important;
+  }
+}

scss/mixins/_buttons.scss

@@ -5,23 +5,23 @@
 
 @mixin button-variant($background, $border, $hover-background: darken($background, 7.5%), $hover-border: darken($border, 10%), $active-background: darken($background, 10%), $active-border: darken($border, 12.5%)) {
   color: color-yiq($background);
-  background-color: $background;
+  @include gradient-bg($background);
   border-color: $border;
   @include box-shadow($btn-box-shadow);
 
   @include hover {
     color: color-yiq($hover-background);
-    background-color: $hover-background;
+    @include gradient-bg($hover-background);
     border-color: $hover-border;
   }
 
   &:focus,
   &.focus {
     // Avoid using mixin so we can pass custom focus shadow properly
     @if $enable-shadows {
-      box-shadow: $btn-box-shadow, 0 0 0 3px rgba($border, .5);
+      box-shadow: $btn-box-shadow, 0 0 0 $input-btn-focus-width rgba($border, .5);
     } @else {
-      box-shadow: 0 0 0 3px rgba($border, .5);
+      box-shadow: 0 0 0 $input-btn-focus-width rgba($border, .5);
     }
   }
 
@@ -35,10 +35,19 @@
   &:not([disabled]):not(.disabled):active,
   &:not([disabled]):not(.disabled).active,
   .show > &.dropdown-toggle {
+    color: color-yiq($active-background);
     background-color: $active-background;
-    background-image: none; // Remove the gradient for the pressed/active state
+    @if $enable-gradients {
+      background-image: none; // Remove the gradient for the pressed/active state
+    }
     border-color: $active-border;
-    @include box-shadow($btn-active-box-shadow);
+
+    // Avoid using mixin so we can pass custom focus shadow properly
+    @if $enable-shadows {
+      box-shadow: $btn-active-box-shadow, 0 0 0 $input-btn-focus-width rgba($border, .5);
+    } @else {
+      box-shadow: 0 0 0 $input-btn-focus-width rgba($border, .5);
+    }
   }
 }
 
@@ -56,7 +65,7 @@
 
   &:focus,
   &.focus {
-    box-shadow: 0 0 0 3px rgba($color, .5);
+    box-shadow: 0 0 0 $input-btn-focus-width rgba($color, .5);
   }
 
   &.disabled,
@@ -71,6 +80,8 @@
     color: $color-hover;
     background-color: $color;
     border-color: $color;
+    // Avoid using mixin so we can pass custom focus shadow properly
+    box-shadow: 0 0 0 $input-btn-focus-width rgba($color, .5);
   }
 }
 

scss/mixins/_forms.scss

@@ -16,7 +16,12 @@
     background-color: $input-focus-bg;
     border-color: $input-focus-border-color;
     outline: none;
-    @include box-shadow($input-focus-box-shadow);
+    // Avoid using mixin so we can pass custom focus shadow properly
+    @if $enable-shadows {
+      box-shadow: $input-box-shadow, $input-btn-focus-box-shadow;
+    } @else {
+      box-shadow: $input-btn-focus-box-shadow;
+    }
   }
 }
 

scss/mixins/_gradients.scss

@@ -1,5 +1,13 @@
 // Gradients
 
+@mixin gradient-bg($color) {
+  @if $enable-gradients {
+    background: $color linear-gradient(180deg, mix($body-bg, $color, 15%), $color) repeat-x;
+  } @else {
+    background-color: $color;
+  }
+}
+
 // Horizontal gradient, from left to right
 //
 // Creates two color stops, start and end, by specifying a color and position for each color stop.

scss/utilities/_background.scss

@@ -4,6 +4,12 @@
   @include bg-variant(".bg-#{$color}", $value);
 }
 
+@if $enable-gradients {
+  @each $color, $value in $theme-colors {
+    @include bg-gradient-variant(".bg-gradient-#{$color}", $value);
+  }
+}
+
 .bg-white {
   background-color: $white !important;
 }