Fix for invalid state in JsonBlockValueConverter when an unused layout has a nested array (#19363)

* Fix for invalid state in JsonBlockValueConverter when an unused layout has a nested array

* Improved comments as suggested by copilot review, also fixed code style miss

* Added check for malformed JSON with more closing array tokens then opening tokens

Author: karl-sjogren

src/Umbraco.Infrastructure/Serialization/JsonBlockValueConverter.cs

@@ -189,9 +189,36 @@ private void DeserializeAndSetLayout(ref Utf8JsonReader reader, JsonSerializerOp
                 else
                 {
                     // ignore this layout - forward the reader to the end of the array and look for the next one
-                    while (reader.TokenType is not JsonTokenType.EndArray)
+
+                    // Read past the current StartArray token before we start counting
+                    _ = reader.Read();
+
+                    // Keep track of the number of open arrays to ensure we find the correct EndArray token
+                    var openCount = 0;
+                    while (true)
                     {
-                        reader.Read();
+                        if (reader.TokenType is JsonTokenType.EndArray && openCount == 0)
+                        {
+                            break;
+                        }
+
+                        if (reader.TokenType is JsonTokenType.StartArray)
+                        {
+                            openCount++;
+                        }
+                        else if (reader.TokenType is JsonTokenType.EndArray)
+                        {
+                            openCount--;
+                            if (openCount < 0)
+                            {
+                                throw new JsonException($"Malformed JSON: Encountered more closing array tokens than opening ones while processing block editor alias: {blockEditorAlias}.");
+                            }
+                        }
+
+                        if (!reader.Read())
+                        {
+                            throw new JsonException($"Unexpected end of JSON while looking for the end of the layout items array for block editor alias: {blockEditorAlias}.");
+                        }
                     }
                 }
             }

tests/Umbraco.Tests.UnitTests/Umbraco.Infrastructure/Serialization/JsonBlockValueConverterTests.cs

@@ -441,4 +441,33 @@ public void Ignores_Other_Layouts()
             Assert.AreEqual(settingsElementKey1, layoutItems.First().SettingsKey);
         });
     }
+
+    [Test]
+    public void Try_Deserialize_Unknown_Block_Layout_With_Nested_Array()
+    {
+        var json = """
+        {
+            "layout": {
+                "Umbraco.BlockGrid": [{
+                        "contentUdi": "umb://element/1304E1DDAC87439684FE8A399231CB3D",
+                        "rowSpan": 1,
+                        "areas": [],
+                        "columnSpan": 12
+                    }
+                ],
+                "Umbraco.BlockList": [{
+                        "contentUdi": "umb://element/1304E1DDAC87439684FE8A399231CB3D"
+                    }
+                ],
+                "Some.Custom.BlockEditor": [{
+                        "contentUdi": "umb://element/1304E1DDAC87439684FE8A399231CB3D"
+                    }
+                ]
+            }
+        }
+""";
+
+        var serializer = new SystemTextJsonSerializer();
+        Assert.DoesNotThrow(() => serializer.Deserialize<BlockListValue>(json));
+    }
 }