Stabilize visible render item sorting

tests/Rendering/unit/test_render_scene_utility.cpp

@@ -11,6 +11,8 @@
 
 #include <cmath>
 #include <memory>
+#include <new>
+#include <type_traits>
 #include <vector>
 
 namespace {
@@ -24,7 +26,9 @@ using XCEngine::Components::Scene;
 using XCEngine::Math::Matrix4x4;
 using XCEngine::Math::Vector3;
 using XCEngine::Rendering::BuildRenderCameraData;
+using XCEngine::Rendering::CompareVisibleRenderItemsStable;
 using XCEngine::Rendering::CollectRenderItemsForEntityIds;
+using XCEngine::Rendering::VisibleRenderItem;
 using XCEngine::Resources::Mesh;
 using XCEngine::Resources::MeshSection;
 using XCEngine::Resources::StaticMeshVertex;
@@ -211,4 +215,33 @@ TEST_F(RenderSceneUtilityTest, CollectRenderItemsForEntityIdsExpandsMeshSections
     EXPECT_EQ(renderables[1].materialIndex, 1u);
 }
 
+TEST(RenderSceneUtilityStandaloneTest, CompareVisibleRenderItemsUsesStableObjectIdOrdering) {
+    using Storage = std::aligned_storage_t<sizeof(GameObject), alignof(GameObject)>;
+
+    Storage storage[2];
+    GameObject* lowerIdHigherAddress = new (&storage[1]) GameObject("LowerId");
+    GameObject* higherIdLowerAddress = new (&storage[0]) GameObject("HigherId");
+
+    ASSERT_LT(lowerIdHigherAddress->GetID(), higherIdLowerAddress->GetID());
+    ASSERT_GT(
+        reinterpret_cast<uintptr_t>(lowerIdHigherAddress),
+        reinterpret_cast<uintptr_t>(higherIdLowerAddress));
+
+    VisibleRenderItem lhs = {};
+    lhs.gameObject = lowerIdHigherAddress;
+    lhs.renderQueue = 2000;
+    lhs.cameraDistanceSq = 9.0f;
+
+    VisibleRenderItem rhs = {};
+    rhs.gameObject = higherIdLowerAddress;
+    rhs.renderQueue = 2000;
+    rhs.cameraDistanceSq = 9.0f;
+
+    EXPECT_TRUE(CompareVisibleRenderItemsStable(lhs, rhs));
+    EXPECT_FALSE(CompareVisibleRenderItemsStable(rhs, lhs));
+
+    higherIdLowerAddress->~GameObject();
+    lowerIdHigherAddress->~GameObject();
+}
+
 } // namespace