diff --git a/app/assets/stylesheets/desktop/topic-post.scss b/app/assets/stylesheets/desktop/topic-post.scss
index d404169fd8f..1b60137985e 100644
--- a/app/assets/stylesheets/desktop/topic-post.scss
+++ b/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 */