Worked on enhancing the JetBrainsRuntime repository by expanding and modernizing test coverage for CharBuffer as CharSequence, migrating legacy tests to JUnit 5 for improved maintainability and reliability. Refactored the CharBuffer test suite to clarify test logic, improve readability, and update bug references, supporting better long-term maintenance. Addressed encoding robustness by fixing a UTF-8 encoding bug for Latin-1 characters in Java, ensuring correct handling of negative bytes and optimizing performance by avoiding unnecessary cloning. Leveraged Core Java, Java NIO, and unit testing practices to deliver more robust, maintainable code and reduce regression risk in string manipulation and encoding functionality.