Fix border on input group elements and custom form inputs

c3e4cbd changed the border of input elements to be translucent. This commit makes
input group elements and custom form inputs follow the same pattern.

Fixes #21098

scss/_custom-forms.scss

@@ -140,6 +140,7 @@
   color: $custom-select-color;
   vertical-align: middle;
   background: $custom-select-bg $custom-select-indicator no-repeat right $custom-select-padding-x center;
+  background-clip: padding-box;
   background-size: $custom-select-bg-size;
   border: $custom-select-border-width solid $custom-select-border-color;
   @include border-radius($custom-select-border-radius);
@@ -216,11 +217,13 @@
   z-index: 5;
   height: $custom-file-height;
   padding: $custom-file-padding-x $custom-file-padding-y;
+  overflow: hidden;
   line-height: $custom-file-line-height;
   color: $custom-file-color;
   pointer-events: none;
   user-select: none;
   background-color: $custom-file-bg;
+  background-clip: padding-box;
   border: $custom-file-border-width solid $custom-file-border-color;
   @include border-radius($custom-file-border-radius);
   @include box-shadow($custom-file-box-shadow);
@@ -233,18 +236,16 @@
 
   &::before {
     position: absolute;
-    top: -$custom-file-border-width;
-    right: -$custom-file-border-width;
-    bottom: -$custom-file-border-width;
+    top: 0;
+    right: 0;
     z-index: 6;
     display: block;
     height: $custom-file-height;
     padding: $custom-file-padding-x $custom-file-padding-y;
     line-height: $custom-file-line-height;
     color: $custom-file-button-color;
     background-color: $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);
+    border-left: $custom-file-border-width solid $custom-file-border-color;
   }
 
   @each $lang, $text in map-get($custom-file-text, button-label) {

scss/_input-group.scss

@@ -74,6 +74,7 @@
   color: $input-color;
   text-align: center;
   background-color: $input-group-addon-bg;
+  background-clip: padding-box;
   border: $input-btn-border-width solid $input-group-addon-border-color;
   @include border-radius($input-border-radius);
 
@@ -144,6 +145,8 @@
   // element above the siblings.
   > .btn {
     position: relative;
+    background-clip: padding-box;
+    border: $input-btn-border-width solid $input-group-btn-border-color;
 
     + .btn {
       margin-left: (-$input-btn-border-width);

scss/_variables.scss

@@ -431,6 +431,7 @@ $form-group-margin-bottom:       1rem !default;
 
 $input-group-addon-bg:           $gray-lighter !default;
 $input-group-addon-border-color: $input-border-color !default;
+$input-group-btn-border-color:   $input-border-color !default;
 
 $custom-control-gutter:   1.5rem !default;
 $custom-control-spacer-y: .25rem !default;