Fix discarded frames during the transition to one frame ahead prediction

Author: MortimerGoro

app/src/main/cpp/BrowserWorld.cpp

@@ -1421,28 +1421,33 @@ BrowserWorld::TickImmersive() {
   m.externalVR->SetCompositorEnabled(false);
   m.device->SetRenderMode(device::RenderMode::Immersive);
 
-  DeviceDelegate::FramePrediction prediction = DeviceDelegate::FramePrediction::NO_FRAME_AHEAD;
-  if (m.device->SupportsFramePrediction(DeviceDelegate::FramePrediction::ONE_FRAME_AHEAD)
-      && m.externalVR->GetVRState() == ExternalVR::VRState::Rendering) {
-    prediction = DeviceDelegate::FramePrediction::ONE_FRAME_AHEAD;
-  }
-
+  const bool supportsFrameAhead = m.device->SupportsFramePrediction(DeviceDelegate::FramePrediction::ONE_FRAME_AHEAD);
   VRB_GL_CHECK(glDepthMask(GL_FALSE));
-  if (prediction == DeviceDelegate::FramePrediction::NO_FRAME_AHEAD) {
-      m.device->StartFrame(prediction);
+  if (!supportsFrameAhead || (m.externalVR->GetVRState() != ExternalVR::VRState::Rendering)) {
+      // Do not use one frame ahead prediction if not supported or we are rendering the spinner.
+      m.device->StartFrame(DeviceDelegate::FramePrediction::NO_FRAME_AHEAD);
       m.externalVR->PushFramePoses(m.device->GetHeadTransform(), m.controllers->GetControllers(),
                                    m.context->GetTimestamp());
   }
   int32_t surfaceHandle, textureWidth, textureHeight = 0;
   device::EyeRect leftEye, rightEye;
   bool aDiscardFrame = !m.externalVR->WaitFrameResult();
   m.externalVR->GetFrameResult(surfaceHandle, textureWidth, textureHeight, leftEye, rightEye);
-  if (prediction == DeviceDelegate::FramePrediction::ONE_FRAME_AHEAD) {
-      m.device->StartFrame(prediction);
+  ExternalVR::VRState state = m.externalVR->GetVRState();
+  if (supportsFrameAhead) {
+      if (m.externalVR->WasFirstPresentingFrame()) {
+          // StartFrame() has been already called to render the spinner, do not call it again.
+          // Instead, repeat the XR frame and render the spinner while we transition
+          // to one frame ahead prediction.
+          state = ExternalVR::VRState::Loading;
+      } else {
+          // Predict poses for one frame ahead and push the data to shmem so Gecko
+          // can start the next XR RAF ASAP.
+          m.device->StartFrame(DeviceDelegate::FramePrediction::ONE_FRAME_AHEAD);
+      }
       m.externalVR->PushFramePoses(m.device->GetHeadTransform(), m.controllers->GetControllers(),
               m.context->GetTimestamp());
   }
-  ExternalVR::VRState state = m.externalVR->GetVRState();
   if (state == ExternalVR::VRState::Rendering) {
     if (!aDiscardFrame) {
       if (textureWidth > 0 && textureHeight > 0) {

app/src/main/cpp/ExternalVR.cpp

@@ -127,6 +127,7 @@ struct ExternalVR::State {
   vrb::Vector eyeOffsets[device::EyeCount];
   uint64_t lastFrameId = 0;
   bool firstPresentingFrame = false;
+  bool wasFirstPresentingFrame = false;
   bool compositorEnabled = false;
   bool waitingForExit = false;
 
@@ -455,6 +456,7 @@ ExternalVR::WaitFrameResult() {
   m.PullBrowserStateWhileLocked();
   while (true) {
     if (!IsPresenting() || m.browser.layerState[0].layer_stereo_immersive.frameId != m.lastFrameId) {
+      m.wasFirstPresentingFrame = m.firstPresentingFrame;
       m.firstPresentingFrame = false;
       m.system.displayState.lastSubmittedFrameSuccessful = true;
       m.system.displayState.lastSubmittedFrameId = m.browser.layerState[0].layer_stereo_immersive.frameId;
@@ -480,6 +482,10 @@ ExternalVR::WaitFrameResult() {
   return true;
 }
 
+bool ExternalVR::WasFirstPresentingFrame() const {
+  return m.wasFirstPresentingFrame;
+}
+
 void
 ExternalVR::CompleteEnumeration()
 {

app/src/main/cpp/ExternalVR.h

@@ -55,6 +55,7 @@ class ExternalVR : public ImmersiveDisplay {
   VRState GetVRState() const;
   void PushFramePoses(const vrb::Matrix& aHeadTransform, const std::vector<Controller>& aControllers, const double aTimestamp);
   bool WaitFrameResult();
+  bool WasFirstPresentingFrame() const;
   void GetFrameResult(int32_t& aSurfaceHandle,
                       int32_t& aTextureWidth,
                       int32_t& aTextureHeight,