[iOS] leak a single overlay layer. (#54189)

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 flutter/flutter#150646 is fixed.

Fixes flutter/flutter#152266

Author: Jonah Williams

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

@@ -158,7 +158,15 @@ 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);
+  if (erase_offset < layers_.size()) {
+    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