diff --git a/notebook/static/notebook/less/cell.less b/notebook/static/notebook/less/cell.less
index 44168224c..978dff6c7 100644
--- a/notebook/static/notebook/less/cell.less
+++ b/notebook/static/notebook/less/cell.less
@@ -27,6 +27,7 @@ div.cell {
         border-left-color: @marked_border_color;
         border-left-width: 3px;
         margin-left: -2px;
+        width: ~"calc(100% + 2px)";
     }
 
     width: 100%;