Worked on the zsh-users/zsh repository to improve shell job control correctness and POSIX wait behavior, focusing on reliability and standards compliance. Addressed a bug where internal jobs could be misidentified as the previous job, refining exit status handling for the 'wait %%' command in POSIX mode. Developed and integrated a regression test to ensure that 'fg %-' with superjobs consistently selects the correct job, preventing future regressions in complex job trees. The work involved deep engagement with C, shell scripting, and systems programming, resulting in more predictable job control behavior and a smoother user experience for shell users.