Nikola Jovicevic enhanced window function processing in the apache/spark repository by refactoring validation logic for modularity and reuse, focusing on SQL windowed operations. Using Scala and Spark, Nikola implemented modular validation for window functions, enabling integration with the single-pass analyzer and reducing runtime validation errors. He updated the WindowResolution component to return WindowExpression objects, streamlining analysis and improving maintainability. Additionally, Nikola introduced timezone-aware casting in WindowFrame type coercion, ensuring that session timezones are respected and minimizing unnecessary changes after WindowSpecDefinition resolution. This work improved correctness, code reuse, and reliability of query results in big data processing environments.