fix compose index error

#425 (comment)

Author: pca006132

src/manifold/src/csg_tree.cpp

@@ -221,13 +221,13 @@ Manifold::Impl CsgLeafNode::Compose(
               combined.halfedge_.begin() + edgeIndices[i],
               UpdateHalfedge({vertIndices[i], edgeIndices[i], triIndices[i]}));
           const bool invert = glm::determinant(glm::mat3(node->transform_)) < 0;
-          for_each_n(
-              policy,
-              zip(combined.halfedgeTangent_.begin(), countAt(edgeIndices[i])),
-              node->pImpl_->halfedgeTangent_.size(),
-              TransformTangents{glm::mat3(node->transform_), invert,
-                                node->pImpl_->halfedgeTangent_.cptrD(),
-                                node->pImpl_->halfedge_.cptrD()});
+          for_each_n(policy,
+                     zip(combined.halfedgeTangent_.begin() + edgeIndices[i],
+                         countAt(0)),
+                     node->pImpl_->halfedgeTangent_.size(),
+                     TransformTangents{glm::mat3(node->transform_), invert,
+                                       node->pImpl_->halfedgeTangent_.cptrD(),
+                                       node->pImpl_->halfedge_.cptrD()});
           if (invert)
             for_each_n(policy,
                        zip(combined.meshRelation_.triRef.begin(),

test/manifold_test.cpp

@@ -1088,3 +1088,12 @@ TEST(Manifold, Invalid) {
   EXPECT_EQ(Manifold::Extrude(empty_circ, 10.).Status(), invalid);
   EXPECT_EQ(Manifold::Revolve(empty_sq).Status(), invalid);
 }
+
+TEST(Manifold, MultiCompose) {
+  auto legAssembly = Manifold::Compose({Manifold::Cube({100, 140, 38})});
+  auto finalAssembly = Manifold::Compose(
+      {legAssembly, legAssembly.Translate({0, 300, 0}),
+       legAssembly.Mirror({1, 0, 0}).Translate({300, 0, 0}),
+       legAssembly.Mirror({1, 0, 0}).Translate({300, 300, 0})});
+  finalAssembly.GetMesh();
+}