handle small desktop height and long menu dropdown

assets/sass/_theme/design-system/header.sass

@@ -50,6 +50,7 @@ header#document-header
         html.has-menu-opened &
             nav
                 padding-bottom: 0
+
 // TODO : Est-ce au bon endroit ?
 body
     &::after

assets/sass/_theme/design-system/nav.sass

@@ -53,6 +53,8 @@
         @include media-breakpoint-up(desktop)
             padding: $spacing1
             position: absolute
+            max-height: calc(100vh - var(--header-height))
+            overflow: auto
         a
             color: $header-dropdown-color 
             &:hover,