Two mocked tests drive _legacy_kill_process_tree down its shared-group
branch with a MagicMock proc whose pid is 999. That branch snapshots
psutil.Process(proc.pid).children(recursive=True) and SIGKILLs each
descendant, and psutil was never patched: the tests killed the real
descendants of whatever the machine runs as PID 999. On macOS runners
PIDs are allocated system-wide, and when 999 was an ancestor of the
test worker the file died mid-run (CRASHED, killed by SIGKILL), which
is how it surfaced on a PR whose diff never touches the browser tool.
Both tests now run against a fake process table, and the shared-group
test asserts the snapshotted descendant is killed individually (the
contract it was written for).
Live repro: inside an unprivileged user+PID namespace with PID 999 an
ancestor of the test runner, main's file reports CRASHED (killed by
SIGKILL); with this change it passes 9/9.
b82d35571c