Fixing tabs and menu alignment.

src/application/menus.less

@@ -32,7 +32,7 @@
 
 
 .p-MenuBar-menu {
-  transform: translateY(0px);
+  transform: translateY(calc(-1*var(--jp-borderWidth)));
 }
 
 

src/application/tabs.less

@@ -7,7 +7,8 @@
 | Variables
 |----------------------------------------------------------------------------*/
 
-.p-TabBar {
+:root {
+  // These need to be root because tabs get attached to the body during dragging.
   --jp-private-horizontal-tab-height: 24px;
   --jp-private-horizontal-tab-width: 144px;
 }