Explicitly make pane active

Previously it relied on focus firing an event which will not occur
when not on the DOM.

src/pane-container.coffee

@@ -113,7 +113,8 @@ class PaneContainer extends View
     @empty()
     if root?
       @append(root)
-      @itemAdded(root.activeItem) if root.activeItem
+      @itemAdded(root.activeItem) if root.activeItem?
+      root.makeActive?()
     @state.set(root: root?.getState())
 
   removeChild: (child) ->

src/pane.coffee

@@ -412,6 +412,7 @@ class Pane extends View
       when 'before' then parent.insertChildBefore(this, newPane)
       when 'after' then parent.insertChildAfter(this, newPane)
     @getContainer().adjustPaneDimensions()
+    newPane.makeActive()
     newPane.focus()
     newPane