Do not rasterize wrong size layer tree after viewport metrics change

shell/common/shell.cc

@@ -832,6 +832,12 @@ void Shell::OnPlatformViewSetViewportMetrics(const ViewportMetrics& metrics) {
           engine->SetViewportMetrics(metrics);
         }
       });
+
+  {
+    std::scoped_lock<std::mutex> lock(resize_mutex_);
+    expected_frame_size_ =
+        SkISize::Make(metrics.physical_width, metrics.physical_height);
+  }
 }
 
 // |PlatformView::Delegate|
@@ -1021,13 +1027,19 @@ void Shell::OnAnimatorDraw(fml::RefPtr<Pipeline<flutter::LayerTree>> pipeline,
     }
   }
 
+  auto discard_callback = [this](flutter::LayerTree& tree) {
+    std::scoped_lock<std::mutex> lock(resize_mutex_);
+    return !expected_frame_size_.isEmpty() &&
+           tree.frame_size() != expected_frame_size_;
+  };
+
   task_runners_.GetRasterTaskRunner()->PostTask(
       [&waiting_for_first_frame = waiting_for_first_frame_,
        &waiting_for_first_frame_condition = waiting_for_first_frame_condition_,
-       rasterizer = rasterizer_->GetWeakPtr(),
-       pipeline = std::move(pipeline)]() {
+       rasterizer = rasterizer_->GetWeakPtr(), pipeline = std::move(pipeline),
+       discard_callback = std::move(discard_callback)]() {
         if (rasterizer) {
-          rasterizer->Draw(pipeline);
+          rasterizer->Draw(pipeline, std::move(discard_callback));
 
           if (waiting_for_first_frame.load()) {
             waiting_for_first_frame.store(false);

shell/common/shell.h

@@ -425,6 +425,13 @@ class Shell final : public PlatformView::Delegate,
   // and read from the raster thread.
   std::atomic<float> display_refresh_rate_ = 0.0f;
 
+  // protects expected_frame_size_ which is set on platform thread and read on
+  // raster thread
+  std::mutex resize_mutex_;
+
+  // used to discard wrong size layer tree produced during interactive resizing
+  SkISize expected_frame_size_ = SkISize::MakeEmpty();
+
   // How many frames have been timed since last report.
   size_t UnreportedFramesCount() const;