Make header more natural. (facebook#10580)

Summary:
Fixed facebook#10381 for blog's navigation bar UI.

Pull Request resolved: facebook#10580

Reviewed By: hx235

Differential Revision: D39079045

Pulled By: cbi42

fbshipit-source-id: 922cf2624f201c0af42815b23d97361fc0151d93

docs/_sass/_header.scss

@@ -3,7 +3,8 @@
   color: $primary-overlay;
   height: $header-height;
   padding: $header-ptop 0 $header-pbot;
-  position: fixed;
+  position: sticky;
+  top: 0;
   width: 100%;
   z-index: 9999;
 
@@ -135,4 +136,4 @@
       max-width: 1400px;
     }
   }
- */
+ */