Set overflow-y: hidden on navbar

In some unusual circumstances, parts of the navbar can overflow
vertically. This interferes with the main body content in surprising
ways, so we want to prevent that.

templates/style/_navbar.scss

@@ -28,6 +28,12 @@ div.nav-container {
     color: var(--color-navbar-standard);
     /* The font size must be specified in pixels because the height is specified in pixels. */
     font: 16px $font-family-sans;
+    /* In some unusual situations, like a locally installed copy of "Fira Sans," elements
+       of the navbar might overflow vertically and start interfering with the main body
+       content. To ensure that doesn't happen, hide any vertical overflow.
+       See https://github.com/rust-lang/docs.rs/issues/1669.
+       */
+    overflow-y: hidden;
 
     li {
         border-left: 1px solid var(--color-border);