Worked on the openai/codex repository to enhance UI responsiveness during complex startup and agent turn transitions. Addressed a critical bug by implementing a derived UI-busy state in Rust, which independently tracked both MCP startup and agent turn progress. This approach ensured that the UI spinner and Esc interrupt hint remained visible during overlapping transitions, preventing premature clearing and reducing user confusion. Focused on concurrency management and event handling to maintain consistent feedback throughout the process. The solution improved the perceived responsiveness of the interface, particularly during startup and turn sequences, and contributed to a smoother user experience without introducing new features.