Dropdown menu should be higher than handle

If the dropdown menu is open over the pane resize handle, the menu will unexpectedly close when the mouse reaches the handle.

This fix bumps the z-index back down (a change was made that increased the `z-index` of `.handle`) below that of  `.dropdown-menu`.

app/styles/_output-pane.scss

@@ -28,7 +28,7 @@
   background-color: transparent;
   position: relative;
   left: -2px;
-  z-index: 1000;
+  z-index: 999;
 
   @media (min-width: $screen-md-min) {
     height: 100%;