FIX: ensure .gap width does not exceed window width

Fixes an issue where the viewport was horizontally scrollable on iPad.

app/assets/stylesheets/desktop/topic-post.scss

@@ -633,6 +633,7 @@ blockquote {
   width: calc(
     #{$topic-avatar-width} + #{$topic-body-width} + 2 * #{$topic-body-width-padding}
   );
+  box-sizing: border-box;
 }
 
 /* hide the reply border above the time gap notices */