On X11 a window's PID comes from the optional _NET_WM_PID property, so
cua-driver's list_windows legitimately returns pid: null for windows that
don't set it (desktop root, panels, override-redirect popups). capture()
and focus_app() coerced every entry via int(w["pid"]) inside a list
comprehension, so a single null-pid window raised TypeError and aborted
the whole enumeration before any screenshot — capture was impossible on
any X11 desktop with even one such window.
Route both ingestion sites through a new _ingest_windows() helper that
skips entries lacking a usable pid/window_id (uncapturable anyway) and
coerces the rest.
Adds tests/tools/test_computer_use_null_pid_windows.py covering the
helper's filtering/coercion and an end-to-end capture() regression that
reproduced the crash.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
