diff --git a/packages/neos-ui/src/Containers/FlashMessages/FlashMessage/style.module.css b/packages/neos-ui/src/Containers/FlashMessages/FlashMessage/style.module.css
index a5a6bc1f2e..d7b8f4c6c8 100644
--- a/packages/neos-ui/src/Containers/FlashMessages/FlashMessage/style.module.css
+++ b/packages/neos-ui/src/Containers/FlashMessages/FlashMessage/style.module.css
@@ -39,6 +39,7 @@
     margin: 0;
     vertical-align: text-top;
     border-right: 1px solid rgba(255, 255, 255, .3);
+    box-sizing: border-box !important;
 }
 
 .flashMessage__heading {