Docs for responsive positioning

modules/primer-marketing-utilities/docs/layout.md

@@ -0,0 +1,21 @@
+---
+title: Layout
+status: New release
+status_issue: https://github.com/github/design-systems/issues/325
+---
+
+The following layout utilities are meant to used in addition to those within primer-core.
+
+{:toc}
+
+## Responsive position
+
+These position utilities behave the same way as the positioning utilities in primer-core, however they include responsive states so that they can be used at specific viewport widths.
+
+```html
+<div class="position-relative p-6">
+  <div class="position-md-absolute bottom-0 right-0 border p-2">
+    <a href="#url" class="d-block p-1">Mashed potatoes</a>
+  </div>
+</div>
+```

modules/primer-marketing-utilities/lib/layout.scss

@@ -9,5 +9,7 @@
     .position-#{$breakpoint}-relative { position: relative !important; }
     /* Set position to absolute */
     .position-#{$breakpoint}-absolute { position: absolute !important; }
+    /* Set position to fixed */
+    .position-#{$breakpoint}-fixed    { position: fixed !important; }
   }
 }