Setting active tab blue top to proper width.

src/application/tabs.css

@@ -259,7 +259,7 @@
   left: calc(-1 * var(--jp-border-width));
   content: '';
   height: var(--jp-private-horizontal-tab-active-top-border);
-  width: var(--jp-private-horizontal-tab-width);
+  width: calc(100% + 2*var(--jp-border-width));
   background: var(--jp-brand-color1);
   z-index: 3;
 }