(pty): a detached child of a Git bash tab survives tab close #1015

Symptom

Found during a live test of main 259011f. #941 ends the processes a tab started when the tab closes. Under PowerShell, a detached child and a foreground child both died within 3 s. Under Git bash, the default shell on this machine, the foreground child died. The detached child (spawn(..., {detached: true, windowsHide: true})) was still alive more than 10 s later, and this happened in two separate runs.

Likely cause (not verified)

MSYS/Cygwin start their children with CREATE_BREAKAWAY_FROM_JOB, and the job allows breakaway (windows-job-helper.ps1 sets limit 0x800, JOB_OBJECT_LIMIT_BREAKAWAY_OK). Such a child therefore leaves the tab's job.

To decide

Choose between two options:

• Drop BREAKAWAY_OK, if nothing relies on it.
• Also walk the process tree when the tab closes.

Measure which processes escape before choosing.