improve scrollbar detection and scroll behavior

src/node.h

@@ -224,6 +224,8 @@ public:
     virtual void loaded();
     virtual void added(Node* parent);
     virtual void removed(Node* parent);
+    virtual void on_child_added(Node* child) { };
+    virtual void on_child_removed(Node* child) { };
     const Node* init_template(const char* id);
     void tick(float dt);
     void app_redraw();