Auto merge of #23891 - gterzian:fix_raf_flodding, r=asajeffrey

WebXr: allow for only a single raf message, until callbacks execute

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See https://github.com/servo/webxr/issues/34

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components/script/dom/xrsession.rs

@@ -285,6 +285,9 @@ impl XRSessionMethods for XRSession {
 
     /// https://immersive-web.github.io/webxr/#dom-xrsession-requestanimationframe
     fn RequestAnimationFrame(&self, callback: Rc<XRFrameRequestCallback>) -> i32 {
+        // We only need to send a message once, until a raf callback executes.
+        let should_send = self.raf_callback_list.borrow().is_empty();
+
         // queue up RAF callback, obtain ID
         let raf_id = self.next_raf_id.get();
         self.next_raf_id.set(raf_id + 1);
@@ -315,10 +318,21 @@ impl XRSessionMethods for XRSession {
                 }),
             );
         }
-        let sender = self.raf_sender.borrow().clone().unwrap();
 
-        // request animation frame
+        if should_send {
+            // If our callback list is empty, it either means this is the first request,
+            // or raf_callback executed, in which case we should
+            // send a message to request an animation frame.
+            //
+            // This prevents multiple messages being sent for a single call to raf_callback,
+            // and multiple message are unnecessary,
+            // since one call will already deal with multiple potentially enqueued callbacks.
+            //
+            // Allowing multiple messages could keep the main-thread,
+            // where the session thread might be running, looping on incoming messages.
+            let sender = self.raf_sender.borrow().clone().unwrap();
             self.session.borrow_mut().request_animation_frame(sender);
+        }
 
         raf_id
     }