Summary

• Fixes orphaned conhost.exe processes remaining after Ctrl+C on Windows (reported in pnpm dev hangs in turborepo-basic with turbo ^2.8.7 (TUI opens, no tasks start) #11808)
• Adds per-child Windows Job Objects with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE so the entire process tree is terminated when a child handle is dropped
• Adds a cross-platform test_process_tree_cleanup regression test that verifies grandchild processes are killed when the parent is stopped

Context

The primary issue from #11808 (ConPTY hanging due to PSEUDOCONSOLE_INHERIT_CURSOR) was fixed in #11816. However, a follow-up report confirmed that conhost.exe processes still remain orphaned after Ctrl+C on Windows.

The root cause: on Unix, setsid() creates a process group and kill(-pgid, SIGINT) kills the entire tree. On Windows, there was no equivalent — TerminateProcess only kills the direct child, leaving conhost.exe (spawned by ConPTY) and any grandchild processes alive.

How it works

Each spawned child process (both normal and PTY) is assigned to a Windows Job Object configured with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE. When the ChildHandle is dropped (after exit or kill), the job handle closes and Windows terminates all remaining processes in the job. This is the standard Windows mechanism for process tree management, analogous to Unix process groups.

Job Object creation failures are logged but non-fatal — the child still runs, just without the cleanup guarantee.

Testing

The new test_process_tree_cleanup test:

1. Spawns a Node.js parent that itself creates a long-lived grandchild
2. Asserts the grandchild is alive (pre-condition)
3. Stops the parent
4. Asserts the grandchild is dead

This test runs on both Unix (validating via setsid + kill(-pgid)) and Windows (validating via Job Objects). It needs to pass on Windows CI to fully confirm the fix.