BUG Fixing "Add new" not adding the page to the correct subsite

_config.php

@@ -7,6 +7,7 @@
 
 SiteTree::add_extension('SiteTreeSubsites');
 ContentController::add_extension('ControllerSubsites');
+CMSPageAddController::add_extension('CMSPageAddControllerExtension');
 LeftAndMain::add_extension('LeftAndMainSubsites');
 LeftAndMain::add_extension('ControllerSubsites');
 

code/extensions/CMSPageAddControllerExtension.php

@@ -0,0 +1,8 @@
+<?php
+class CMSPageAddControllerExtension extends Extension {
+
+	function updatePageOptions(&$fields) {
+		$fields->push(new HiddenField('SubsiteID', 'SubsiteID', Subsite::currentSubsiteID()));
+	}
+
+}