In the demo, handle events before updating the scene

demo/common/src/lib.rs

@@ -184,12 +184,12 @@ impl<W> DemoApp<W> where W: Window {
     }
 
     pub fn prepare_frame(&mut self, events: Vec<Event>) -> u32 {
-        // Update the scene.
-        self.build_scene();
-
         // Handle events.
         let ui_events = self.handle_events(events);
 
+        // Update the scene.
+        self.build_scene();
+
         // Get the render message, and determine how many scenes it contains.
         let render_msg = self.scene_thread_proxy.receiver.recv().unwrap();
         let render_scene_count = render_msg.render_scenes.len() as u32;