diff --git a/app/assets/stylesheets/common/base/header.scss b/app/assets/stylesheets/common/base/header.scss
index 0cc0a38922e..07cffbaf119 100644
--- a/app/assets/stylesheets/common/base/header.scss
+++ b/app/assets/stylesheets/common/base/header.scss
@@ -14,6 +14,7 @@
   backface-visibility: hidden; /** do magic for scrolling performance **/
 
   > .wrap {
+    box-sizing: border-box;
     width: 100%;
     height: 100%;
 
diff --git a/app/assets/stylesheets/desktop/discourse.scss b/app/assets/stylesheets/desktop/discourse.scss
index ccc6e3cd8fc..22ad6fb6be8 100644
--- a/app/assets/stylesheets/desktop/discourse.scss
+++ b/app/assets/stylesheets/desktop/discourse.scss
@@ -184,6 +184,7 @@ input {
 }
 
 #main-outlet-wrapper {
+  box-sizing: border-box;
   width: 100%;
   display: grid;
   grid-template-areas: "content";