[Regression]: Output is truncated when piped

[twk3]

Last Good Version

1.60

First Bad Version

1.61

Steps to reproduce

(POSIX, large suite, 1.61 vs 1.60):

npx playwright test --list --reporter=list > out.txt # tail missing on 1.61
npx playwright test --list --reporter=list | wc -l # fewer lines than "Total: N"

Expected behavior

No missing (truncated) lines in output

Actual behavior

On large suites, depending on system speed, the last few lines of output are truncated and missing from the piped results

Additional context

This is related to the StripAnsiStream that now wraps stdout/stderr introduced in #40688

It's not being flushed on exit, in gracefullyProcessExitDoNotHang so it's possible for the pipe to be closed before all the output has been flushed.

Environment

System:
    OS: macOS 26.5.1
    CPU: (14) arm64 Apple M3 Max
    Memory: 110.64 MB / 36.00 GB
  Binaries:
    Node: 24.14.1 - /Users/dmountney/.nvm/versions/node/v24.14.1/bin/node
    Yarn: 1.22.22 - /Users/dmountney/.nvm/versions/node/v24.14.1/bin/yarn
    npm: 11.11.0 - /Users/dmountney/.nvm/versions/node/v24.14.1/bin/npm
    pnpm: 11.1.2 - /opt/homebrew/bin/pnpm
  IDEs:
    VSCode: 1.104.1 - /opt/homebrew/bin/code
    Cursor: 3.8.23 - /usr/local/bin/cursor
    Claude Code: 2.1.190 - /Users/dmountney/.local/bin/claude
  Languages:
    Bash: 3.2.57 - /bin/bash
  npmPackages:
    @playwright/test: ^1.61.1 => 1.61.1