Arthur Cose developed and refined core editor functionality for the Floem and Vello repositories, focusing on reliability, user experience, and rendering accuracy. He implemented overlay window structure optimizations and enhanced cursor affinity logic, improving multi-selection and caret positioning in Rust-based code editors. His work addressed complex input handling, including IME integration and Unicode support, ensuring stable text input and accurate rendering for diverse scripts. By resolving 18 bugs and delivering 7 targeted features, Arthur improved UI layering, event handling, and graphics rendering. The depth of his contributions is reflected in robust error handling and thoughtful system programming across both projects.