Deprecate the flex-calc mixin.

assets/sass/components/flex-group/_flex-group.scss

@@ -1,9 +1,9 @@
 .flex-group,
 %flex-group {
-  $gap: 1;
+  --flex-gap: calc(var(--base-line-height) * 1rem);
 
-  @include gap($gap);
   display: flex;
+  gap: var(--flex-gap);
   flex-wrap: wrap;
 
   & > * {
@@ -12,27 +12,27 @@
 
   @media (min-width: 666px) {
     &--2 > * {
-      @include flex(2, $gap);
+      flex-basis: calc(100% / 2 - var(--flex-gap));
       flex-grow: 0;
     }
 
     &--3 > * {
-      @include flex(3, $gap);
+      flex-basis: calc(100% / 3 - var(--flex-gap));
       flex-grow: 0;
     }
 
     &--4 > * {
-      @include flex(4, $gap);
+      flex-basis: calc(100% / 4 - var(--flex-gap));
       flex-grow: 0;
     }
 
     &--5 > * {
-      @include flex(5, $gap);
+      flex-basis: calc(100% / 5 - var(--flex-gap));
       flex-grow: 0;
     }
 
     &--6 > * {
-      @include flex(6, $gap);
+      flex-basis: calc(100% / 6 - var(--flex-gap));
       flex-grow: 0;
     }
 

assets/sass/components/hidden/_hidden.scss

@@ -17,13 +17,13 @@
 .js-hidden,
 %js-hidden {
   html.js & {
-    @extend %hidden;
+    display: none;
   }
 }
 
 .nojs-hidden,
 %nojs-hidden {
   html.nojs & {
-    @extend %hidden;
+    display: none;
   }
 }