Remove any empty batches to avoid empty draw calls / batch breaks.

webrender/src/batch.rs

@@ -319,6 +319,24 @@ impl BatchList {
         }
     }
 
+    // Remove any batches that were added but didn't get any instances
+    // added to them.
+    fn remove_unused_batches(&mut self) {
+        if self.opaque_batch_list
+               .batches
+               .last()
+               .map_or(false, |batch| batch.instances.is_empty()) {
+            self.opaque_batch_list.batches.pop().unwrap();
+        }
+
+        if self.alpha_batch_list
+               .batches
+               .last()
+               .map_or(false, |batch| batch.instances.is_empty()) {
+            self.alpha_batch_list.batches.pop().unwrap();
+        }
+    }
+
     fn finalize(&mut self) {
         self.opaque_batch_list.finalize()
     }
@@ -1314,6 +1332,8 @@ impl AlphaBatchBuilder {
                 batch.push(PrimitiveInstance::from(base_instance));
             }
         }
+
+        self.batch_list.remove_unused_batches();
     }
 }
 

webrender/src/renderer.rs

@@ -2601,6 +2601,12 @@ impl Renderer {
         vertex_array_kind: VertexArrayKind,
         stats: &mut RendererStats,
     ) {
+        // If we end up with an empty draw call here, that means we have
+        // probably introduced unnecessary batch breaks during frame
+        // building - so we should be catching this earlier and removing
+        // the batch.
+        debug_assert!(!data.is_empty());
+
         let vao = get_vao(vertex_array_kind, &self.vaos, &self.gpu_glyph_renderer);
 
         self.device.bind_vao(vao);