diff --git a/lib/editor/tinymce/styles.css b/lib/editor/tinymce/styles.css
index b588250f2f1..79bbd60ae6c 100644
--- a/lib/editor/tinymce/styles.css
+++ b/lib/editor/tinymce/styles.css
@@ -18,3 +18,7 @@
         margin-left: -3px;
     }
 }
+
+.mceToolbar td {
+    box-sizing: content-box;
+}