diff --git a/src/gn/command_format.cc b/src/gn/command_format.cc
index 6118fa4..041a037 100644
--- a/src/gn/command_format.cc
+++ b/src/gn/command_format.cc
@@ -170,6 +170,10 @@
   // 'visibility': same as 'deps'.
   void SortIfApplicable(const BinaryOpNode* binop);
 
+  // Traverse a binary op node tree and apply a callback to each leaf node.
+  void TraverseBinaryOpNode(const ParseNode* node,
+                            std::function<void(const ParseNode*)> callback);
+
   // Sort contiguous import() function calls in the given ordered list of
   // statements (the body of a block or scope).
   template <class PARSENODE>
@@ -403,17 +407,37 @@
     }
   }
   const IdentifierNode* ident = binop->left()->AsIdentifier();
-  const ListNode* list = binop->right()->AsList();
   if ((binop->op().value() == "=" || binop->op().value() == "+=" ||
        binop->op().value() == "-=") &&
-      ident && list) {
+      ident) {
     const std::string_view lhs = ident->value().value();
     if (base::EndsWith(lhs, "sources", base::CompareCase::SENSITIVE) ||
-        lhs == "public")
-      const_cast<ListNode*>(list)->SortAsStringsList();
-    else if (base::EndsWith(lhs, "deps", base::CompareCase::SENSITIVE) ||
-             lhs == "visibility")
-      const_cast<ListNode*>(list)->SortAsTargetsList();
+        lhs == "public") {
+      TraverseBinaryOpNode(binop->right(), [](const ParseNode* node) {
+        const ListNode* list = node->AsList();
+        if (list)
+          const_cast<ListNode*>(list)->SortAsStringsList();
+      });
+    } else if (base::EndsWith(lhs, "deps", base::CompareCase::SENSITIVE) ||
+               lhs == "visibility") {
+      TraverseBinaryOpNode(binop->right(), [](const ParseNode* node) {
+        const ListNode* list = node->AsList();
+        if (list)
+          const_cast<ListNode*>(list)->SortAsTargetsList();
+      });
+    }
+  }
+}
+
+void Printer::TraverseBinaryOpNode(
+    const ParseNode* node,
+    std::function<void(const ParseNode*)> callback) {
+  const BinaryOpNode* binop = node->AsBinaryOp();
+  if (binop) {
+    TraverseBinaryOpNode(binop->left(), callback);
+    TraverseBinaryOpNode(binop->right(), callback);
+  } else {
+    callback(node);
   }
 }
 
diff --git a/src/gn/command_format_unittest.cc b/src/gn/command_format_unittest.cc
index 9112dd3..de7e004 100644
--- a/src/gn/command_format_unittest.cc
+++ b/src/gn/command_format_unittest.cc
@@ -136,3 +136,4 @@
 FORMAT_TEST(081)
 FORMAT_TEST(082)
 FORMAT_TEST(083)
+FORMAT_TEST(084)
diff --git a/src/gn/format_test_data/084.gn b/src/gn/format_test_data/084.gn
new file mode 100644
index 0000000..b58b634
--- /dev/null
+++ b/src/gn/format_test_data/084.gn
@@ -0,0 +1,34 @@
+# Checks that lists are also sorted if concatenated during an assignment
+other_sources = [
+  "f.cc",
+  "e.cc"
+] + [
+  "h.cc",
+  "g.cc"
+]
+
+other_sources = [
+  ":f",
+  ":e"
+] + [
+  ":h",
+  "g"
+]
+
+executable("foo") {
+  sources = [
+    "b.cc",
+    "a.cc"
+  ] + [
+    "d.cc",
+    "c.cc"
+  ] + other_sources
+
+  deps = [
+    ":b",
+    ":a"
+  ] + [
+    ":d",
+    ":c"
+  ] + other_sources
+}
diff --git a/src/gn/format_test_data/084.golden b/src/gn/format_test_data/084.golden
new file mode 100644
index 0000000..fa18037
--- /dev/null
+++ b/src/gn/format_test_data/084.golden
@@ -0,0 +1,38 @@
+# Checks that lists are also sorted if concatenated during an assignment
+other_sources = [
+                  "e.cc",
+                  "f.cc",
+                ] +
+                [
+                  "g.cc",
+                  "h.cc",
+                ]
+
+other_sources = [
+                  ":e",
+                  ":f",
+                ] +
+                [
+                  ":h",
+                  "g",
+                ]
+
+executable("foo") {
+  sources = [
+              "a.cc",
+              "b.cc",
+            ] +
+            [
+              "c.cc",
+              "d.cc",
+            ] + other_sources
+
+  deps = [
+           ":a",
+           ":b",
+         ] +
+         [
+           ":c",
+           ":d",
+         ] + other_sources
+}