[iOS] leak a single overlay layer.

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -158,7 +158,13 @@ static bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
   for (size_t i = available_layer_index_; i < layers_.size(); i++) {
     results.push_back(layers_[i]);
   }
-  layers_.erase(layers_.begin() + available_layer_index_, layers_.end());
+  // Leave at least one overlay layer, to work around cases where scrolling
+  // platform views under an app bar continually adds and removes an
+  // overlay layer. This logic could be removed if https://github.com/flutter/flutter/issues/150646
+  // is fixed.
+  static constexpr size_t kLeakLayerCount = 1;
+  size_t erase_offset = std::max(available_layer_index_, kLeakLayerCount);
+  layers_.erase(layers_.begin() + erase_offset, layers_.end());
   return results;
 }
 

shell/platform/darwin/ios/framework/Source/FlutterPlatformViewsTest.mm

@@ -3335,10 +3335,10 @@ - (void)testLayerPool {
   pool.RecycleLayers();
   XCTAssertEqual(pool.size(), 2u);
 
-  // Free the unused layers.
+  // Free the unused layers. One should remain.
   auto unused_layers = pool.RemoveUnusedLayers();
   XCTAssertEqual(unused_layers.size(), 2u);
-  XCTAssertEqual(pool.size(), 0u);
+  XCTAssertEqual(pool.size(), 1u);
 }
 
 @end