Bug 1591754 - Fix incorrect surface deletions when frames are skipped. r=nical

If the render backend is producing frames too quickly for the
renderer thread to consume, old frames are dropped in favor of
the most recent frame.

When this occurs, we need to ensure that any native surface updates
from the skipped frame are also applied. Otherwise, the state of
the native surfaces list can get out of sync between the renderer
and render backend threads.

Differential Revision: https://phabricator.services.mozilla.com/D53122

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/renderer.rs

@@ -2490,7 +2490,16 @@ impl Renderer {
                                 let device_size = self.device_size;
                                 self.render_impl(device_size).ok();
                             }
-                            self.active_documents[pos].1 = doc;
+
+                            let mut old_doc = mem::replace(
+                                &mut self.active_documents[pos].1,
+                                doc,
+                            );
+
+                            // If the document we are overwriting has any pending
+                            // native surface updates, ensure they are flushed
+                            // before replacing with the new document.
+                            self.update_native_surfaces(&mut old_doc.frame.composite_state);
                         }
                         None => self.active_documents.push((document_id, doc)),
                     }
@@ -5008,11 +5017,11 @@ impl Renderer {
 
     fn update_native_surfaces(
         &mut self,
-        composite_state: &CompositeState,
+        composite_state: &mut CompositeState,
     ) {
         match self.compositor_config {
             CompositorConfig::Native { ref mut compositor, .. } => {
-                for op in &composite_state.native_surface_updates {
+                for op in composite_state.native_surface_updates.drain(..) {
                     match op.details {
                         NativeSurfaceOperationDetails::CreateSurface { size, is_opaque } => {
                             let _inserted = self.allocated_native_surfaces.insert(op.id);
@@ -5065,7 +5074,7 @@ impl Renderer {
         self.device.disable_stencil();
 
         self.bind_frame_data(frame);
-        self.update_native_surfaces(&frame.composite_state);
+        self.update_native_surfaces(&mut frame.composite_state);
 
         for (_pass_index, pass) in frame.passes.iter_mut().enumerate() {
             #[cfg(not(target_os = "android"))]