headless: cloak on Windows/macOS, Xvfb on Linux; CI cloak + webgl-masking guards

headless=True now hides the window via the binary's own cloak pref
(zoom.stealth.cloak_windows) on Windows and macOS instead of the broken
thread-level SetThreadDesktop; macOS is now supported. Linux keeps Xvfb.

Adds e2e guards that also run per-platform in the release drive-gate:
- test_cloak: the window is hidden (Windows DWMWA_CLOAKED / macOS CGWindowAlpha)
  yet still renders + drives; the macOS leg is where the cocoa cloak patch runs.
- a WebGL readPixels masking guard: the gamma noise must stay a smooth gamma
  remap, not the pixelscan-maskable +-1 spikes.

.github/workflows/release.yml

@@ -341,6 +341,26 @@ jobs:
         shell: bash
         run: python scripts/ci_drive_gate.py "$FF_EXE" ${{ matrix.extra }}
 
+      # CLOAK + WEBGL-MASKING GUARDS — run the wrapper's e2e cloak/gamma checks
+      # against THIS leg's freshly-built artifact, on its native runner. The
+      # wrapper's headless=True is headed+hidden (cloak on Win/macOS, its own
+      # Xvfb on Linux), so software-GL rendering works on the GPU-less hosts.
+      # test_cloak asserts the window is hidden (Windows DWMWA_CLOAKED / macOS
+      # CGWindowAlpha) AND still renders — the macOS leg is the only place the
+      # cocoa cloak patch gets RUN. The webgl guard catches a regression of the
+      # gamma readPixels noise back to the pixelscan-maskable ±1 spike form.
+      - name: Install pyobjc Quartz (macOS — to read the cloak window alpha)
+        if: matrix.kind == 'mac'
+        run: python -m pip install --quiet pyobjc-framework-Quartz
+      - name: Cloak + WebGL-masking guards (headed)
+        shell: bash
+        run: |
+          python -m pip install --quiet -e .
+          INVPW_BINARY_PATH="$FF_EXE" python -m pytest \
+            tests/test_cloak.py \
+            "tests/test_fingerprint_surface.py::test_webgl_readpixels_no_masking_signature" \
+            -m e2e -o addopts='' -q
+
   publish:
     name: publish-draft-release
     needs: [build, gate]

src/invisible_playwright/_headless.py

@@ -2,18 +2,23 @@
 
 Playwright's ``headless=True`` flips Firefox onto a different code path —
 no widget tree, software-only rendering, distinct timing — and anti-bot
-systems can spot the divergence. Running the browser *headed* on a
-virtual display gives us the real rendering pipeline while keeping the
-windows off the user's screen.
+systems can spot the divergence. Running the browser *headed* but hidden
+gives us the real rendering pipeline while keeping the windows off screen.
 
-Linux: spawns its own ``Xvfb`` instance, points ``DISPLAY`` at it.
-Windows: creates a hidden desktop via ``CreateDesktop`` and binds the
-calling thread to it, so Playwright's child processes inherit it.
+Two mechanisms, by platform:
+
+- **Windows & macOS**: the patched binary cloaks its OWN chrome windows
+  when ``zoom.stealth.cloak_windows`` is set — ``DWMWA_CLOAK`` (Windows)
+  / ``NSWindow`` alpha-0 + pinned occlusion-ignore (macOS). The window
+  renders on the real GPU but never appears on screen, in the taskbar or
+  the Dock. The launcher injects the pref; nothing host-side is spawned.
+
+- **Linux**: spawns its own ``Xvfb`` instance and points ``DISPLAY`` at
+  it (X11/Wayland have no per-window cloak that keeps the GPU rendering).
 """
 from __future__ import annotations
 
 import os
-import secrets
 import subprocess
 import sys
 import time
@@ -131,95 +136,40 @@ class _LinuxVirtualDisplay:
         self._display = None
 
 
-class _WindowsVirtualDesktop:
-    """A hidden Windows desktop the calling thread is bound to.
+# Windows & macOS: the patched Firefox cloaks its own chrome windows when this
+# pref is set (DWMWA_CLOAK / NSWindow alpha-0 + pinned occlusion-ignore), so the
+# window renders on the real GPU but never shows on screen / in the taskbar or
+# Dock. window_occlusion_tracking is disabled so a hidden window keeps painting.
+CLOAK_PREFS = {
+    "zoom.stealth.cloak_windows": True,
+    "widget.windows.window_occlusion_tracking.enabled": False,
+}
 
-    Playwright's child processes (node driver → firefox.exe) inherit the
-    desktop because their ``STARTUPINFO.lpDesktop`` is NULL — Windows uses
-    the calling thread's desktop in that case.
 
-    pywin32 ships ``CreateDesktop`` in ``win32service`` but doesn't expose
-    ``SetThreadDesktop`` / ``GetThreadDesktop`` as module functions. We
-    call them directly via ctypes against ``user32.dll``.
+def cloak_prefs() -> dict:
+    """Prefs that make the patched binary self-cloak its chrome windows.
+
+    Used on Windows & macOS, where hiding is done inside the binary rather than
+    with a host-side virtual display.
     """
-
-    def __init__(self) -> None:
-        self._desktop = None      # PyHDESK from win32service.CreateDesktop
-        self._original_handle = 0  # raw HDESK int of the previous desktop
-
-    def start(self) -> None:
-        try:
-            import win32con  # type: ignore
-            import win32service  # type: ignore
-        except ImportError as e:
-            raise RuntimeError(
-                "invisible_playwright headless=True on Windows requires pywin32. "
-                "Install it: pip install pywin32"
-            ) from e
-
-        import ctypes
-        from ctypes import wintypes
-        user32 = ctypes.windll.user32
-        kernel32 = ctypes.windll.kernel32
-
-        # Save the current desktop handle so we can restore it on stop().
-        get_thread_desktop = user32.GetThreadDesktop
-        get_thread_desktop.argtypes = [wintypes.DWORD]
-        get_thread_desktop.restype  = wintypes.HANDLE
-        self._original_handle = get_thread_desktop(kernel32.GetCurrentThreadId())
-
-        name = f"sf_{secrets.token_hex(4)}"
-        self._desktop = win32service.CreateDesktop(
-            name, 0, win32con.GENERIC_ALL, None
-        )
-
-        # Bind the calling thread to the new desktop. Children spawned
-        # afterwards (Playwright driver → firefox.exe) inherit it because
-        # their STARTUPINFO.lpDesktop is NULL.
-        set_thread_desktop = user32.SetThreadDesktop
-        set_thread_desktop.argtypes = [wintypes.HANDLE]
-        set_thread_desktop.restype  = wintypes.BOOL
-        if not set_thread_desktop(int(self._desktop)):
-            err = ctypes.get_last_error()
-            raise RuntimeError(
-                f"SetThreadDesktop failed (GetLastError={err}). "
-                "The thread cannot have any windows or hooks; close them first."
-            )
-
-    def stop(self) -> None:
-        import ctypes
-        from ctypes import wintypes
-        user32 = ctypes.windll.user32
-
-        if self._original_handle:
-            try:
-                set_thread_desktop = user32.SetThreadDesktop
-                set_thread_desktop.argtypes = [wintypes.HANDLE]
-                set_thread_desktop.restype  = wintypes.BOOL
-                set_thread_desktop(self._original_handle)
-            except Exception:
-                pass
-            self._original_handle = 0
-
-        if self._desktop is not None:
-            try:
-                self._desktop.CloseDesktop()
-            except Exception:
-                pass
-            self._desktop = None
+    return dict(CLOAK_PREFS)
 
 
 def make_virtual_display():
-    """Return a started/stoppable virtual-display object for this platform.
+    """Return a start()/stop()-able virtual display, or ``None`` when the
+    platform hides windows via the in-binary cloak pref instead.
 
-    InvisiblePlaywright supports Windows x86_64 and Linux x86_64 only.
+    - Linux: a fresh ``Xvfb`` (the launcher start()s/stop()s it).
+    - Windows / macOS: ``None`` — the binary self-cloaks via ``cloak_prefs()``,
+      injected by the launcher; nothing host-side needs spawning.
     """
-    if sys.platform == "win32":
-        return _WindowsVirtualDesktop()
     if sys.platform.startswith("linux"):
         return _LinuxVirtualDisplay()
+    if sys.platform in ("win32", "darwin"):
+        return None
     raise RuntimeError(
-        f"invisible_playwright supports Windows and Linux only (got {sys.platform!r})"
+        f"invisible_playwright supports Windows, macOS and Linux "
+        f"(got {sys.platform!r})"
     )
 
 

src/invisible_playwright/launcher.py

@@ -9,7 +9,7 @@ from playwright.sync_api import Browser, BrowserContext, Playwright, sync_playwr
 
 from ._fpforge import Profile, generate_profile
 from ._geo import prepare_session_geo
-from ._headless import make_virtual_display
+from ._headless import cloak_prefs, make_virtual_display
 from ._proxy import configure_proxy as _configure_proxy_shared
 from .download import ensure_binary
 from .prefs import translate_profile_to_prefs
@@ -340,6 +340,12 @@ class InvisiblePlaywright:
             extra_prefs=self._extra_prefs,
             virtual_display=bool(self._headless and _sys.platform == "win32"),
         )
+        # Windows & macOS hide the headless window via the binary's own cloak
+        # (DWMWA_CLOAK / NSWindow alpha) — inject the pref so the patched build
+        # cloaks its chrome windows. setdefault: an explicit user override wins.
+        if self._headless and _sys.platform in ("win32", "darwin"):
+            for _k, _v in cloak_prefs().items():
+                prefs.setdefault(_k, _v)
         prefs["invisible_playwright.humanize"] = bool(self._humanize)
         if self._humanize:
             prefs["invisible_playwright.humanize.maxTime"] = str(self._humanize_max_seconds())
@@ -379,15 +385,18 @@ class InvisiblePlaywright:
     def _resolve_headless(self) -> bool:
         """Translate the user's ``headless`` flag.
 
-        When ``True``, we keep Firefox in headed mode (real rendering
-        pipeline → coherent fingerprint) and hide the windows on a fresh
-        Xvfb (Linux) or hidden Windows desktop.
+        When ``True``, Firefox stays in headed mode (real rendering pipeline →
+        coherent fingerprint) and the window is hidden: on Linux via a fresh
+        Xvfb spawned here; on Windows/macOS via the binary's own window cloak
+        (the ``zoom.stealth.cloak_windows`` pref added in ``_build_prefs``), so
+        ``make_virtual_display()`` returns ``None`` and nothing is spawned.
         """
         if not self._headless:
             return False
         vd = make_virtual_display()
-        vd.start()
-        self._virtual_display = vd
+        if vd is not None:
+            vd.start()
+            self._virtual_display = vd
         return False
 
     def _humanize_max_seconds(self) -> float:

tests/test_cloak.py

@@ -0,0 +1,106 @@
+"""Cloak guard (e2e) — verifies the source-level "invisible headless" cloak:
+the chrome window is hidden from the screen YET keeps rendering on the real GPU
+(not Playwright's native headless, which has no WebGL). Runs per-platform in CI:
+- Windows: the DWMWA_CLOAK attribute (queried via DWMWA_CLOAKED).
+- macOS:   the NSWindow alpha (queried via Quartz CGWindowListCopyWindowInfo).
+- Linux:   skipped — there the wrapper hides via Xvfb, not a source-level cloak.
+
+This is the CI validation for the macOS cocoa cloak patch, which can't be built
+or run on the Windows/Linux dev boxes.
+"""
+from __future__ import annotations
+
+import sys
+import time
+
+import pytest
+
+from invisible_playwright import InvisiblePlaywright
+
+CLOAK_PREFS = {
+    "zoom.stealth.cloak_windows": True,
+    "widget.windows.window_occlusion_tracking.enabled": False,
+}
+
+_WEBGL_RENDERER = """() => {
+  const g = document.createElement('canvas').getContext('webgl');
+  if (!g) return 'NO-WEBGL';
+  const d = g.getExtension('WEBGL_debug_renderer_info');
+  return d ? g.getParameter(d.UNMASKED_RENDERER_WEBGL) : (g.getParameter(g.RENDERER) || '');
+}"""
+
+
+def _windows_moz_window_cloaked() -> bool:
+    """True if at least one MozillaWindowClass top-level window is DWMWA_CLOAKED."""
+    import ctypes
+    from ctypes import wintypes
+
+    user32 = ctypes.windll.user32
+    dwm = ctypes.windll.dwmapi
+    DWMWA_CLOAKED = 14
+    ENUM = ctypes.WINFUNCTYPE(wintypes.BOOL, wintypes.HWND, wintypes.LPARAM)
+    found = []
+
+    def cb(hwnd, _):
+        c = ctypes.create_unicode_buffer(256)
+        user32.GetClassNameW(hwnd, c, 256)
+        if c.value == "MozillaWindowClass":
+            v = wintypes.DWORD(0)
+            dwm.DwmGetWindowAttribute(wintypes.HWND(hwnd), DWMWA_CLOAKED,
+                                      ctypes.byref(v), 4)
+            found.append(v.value)
+        return True
+
+    user32.EnumWindows(ENUM(cb), 0)
+    return any(state != 0 for state in found)
+
+
+def _macos_firefox_window_alpha_zero() -> bool:
+    """True if a Firefox on-screen window reports ~0 alpha (cloaked)."""
+    from Quartz import (  # type: ignore
+        CGWindowListCopyWindowInfo,
+        kCGWindowListOptionOnScreenOnly,
+        kCGNullWindowID,
+    )
+
+    infos = CGWindowListCopyWindowInfo(kCGWindowListOptionOnScreenOnly, kCGNullWindowID)
+    alphas = []
+    for w in infos or []:
+        owner = (w.get("kCGWindowOwnerName") or "")
+        if "firefox" in owner.lower() or "nightly" in owner.lower():
+            alphas.append(float(w.get("kCGWindowAlpha", 1.0)))
+    # cloaked windows are alpha 0; if Firefox has any window it must be ~0.
+    return bool(alphas) and all(a < 0.05 for a in alphas)
+
+
+@pytest.mark.e2e
+@pytest.mark.skipif(
+    sys.platform.startswith("linux"),
+    reason="source-level cloak is Windows/macOS only; Linux hides via Xvfb",
+)
+def test_cloak_hides_window_but_keeps_rendering(firefox_binary):
+    with InvisiblePlaywright(
+        seed=42, binary_path=firefox_binary, headless=False, extra_prefs=CLOAK_PREFS
+    ) as browser:
+        page = browser.new_context().new_page()
+        page.goto("https://example.com", timeout=30_000)
+        time.sleep(2)
+
+        # 1) still renders on the real GPU pipeline (a non-blank screenshot proves
+        #    the compositor is alive despite the window being hidden).
+        shot = page.screenshot()
+        assert len(shot) > 3000, "cloaked window produced a blank screenshot (rendering paused)"
+
+        # 2) real WebGL present (native headless has none) -> headed pipeline intact.
+        renderer = page.evaluate(_WEBGL_RENDERER)
+        assert renderer and renderer != "NO-WEBGL", f"no real WebGL under cloak: {renderer!r}"
+
+        # 3) the window is actually hidden (per-platform).
+        if sys.platform == "win32":
+            assert _windows_moz_window_cloaked(), "Firefox window is not DWMWA_CLOAKED"
+        elif sys.platform == "darwin":
+            try:
+                hidden = _macos_firefox_window_alpha_zero()
+            except ImportError:
+                pytest.skip("pyobjc Quartz not available to verify macOS cloak alpha")
+            assert hidden, "Firefox macOS window is not alpha-cloaked"

tests/test_fingerprint_surface.py

@@ -236,3 +236,69 @@ def test_fpscanner_no_userAgentData_on_firefox(page):
     """navigator.userAgentData is Chromium-only. Presence on Firefox UA = bot."""
     if "Firefox" in _ev(page, "navigator.userAgent"):
         assert not _ev(page, "'userAgentData' in navigator")
+
+
+# ===========================================================================
+# WebGL masking-detector guard (pixelscan getFixedRedBox / webglHash)
+#
+# pixelscan flags "fingerprint masking" on the WebGL readPixels output. We
+# reproduce ITS probe locally (the fingerprintjs gradient triangle) and check
+# the structural signature it keys on: our stealth readPixels noise MUST be a
+# coherent, monotonic gamma remap (smooth, ~0 spikes), NOT isolated +-1 flips
+# (which read as unnatural high-frequency noise and were flagged as masking).
+# This is the CI-safe local stand-in for pixelscan's server-side check; it
+# guards the gamma fix from ever silently regressing to the +-1 algorithm.
+# ===========================================================================
+
+_WEBGL_MASKING_PROBE = """() => {
+  const c = document.createElement('canvas');
+  const gl = c.getContext('webgl') || c.getContext('experimental-webgl');
+  if (!gl) return { error: 'no-webgl' };
+  const vs = 'attribute vec2 a;uniform vec2 o;varying vec2 v;' +
+             'void main(){v=a+o;gl_Position=vec4(a,0,1);}';
+  const fs = 'precision mediump float;varying vec2 v;' +
+             'void main(){gl_FragColor=vec4(v,0,1);}';
+  const buf = gl.createBuffer(); gl.bindBuffer(gl.ARRAY_BUFFER, buf);
+  gl.bufferData(gl.ARRAY_BUFFER,
+    new Float32Array([-0.2,-0.9,0, 0.4,-0.26,0, 0,0.732134444,0]), gl.STATIC_DRAW);
+  const p = gl.createProgram();
+  const s1 = gl.createShader(gl.VERTEX_SHADER); gl.shaderSource(s1, vs); gl.compileShader(s1);
+  const s2 = gl.createShader(gl.FRAGMENT_SHADER); gl.shaderSource(s2, fs); gl.compileShader(s2);
+  gl.attachShader(p, s1); gl.attachShader(p, s2); gl.linkProgram(p); gl.useProgram(p);
+  const loc = gl.getAttribLocation(p, 'a'); gl.enableVertexAttribArray(loc);
+  gl.vertexAttribPointer(loc, 3, gl.FLOAT, false, 0, 0);
+  const off = gl.getUniformLocation(p, 'o'); gl.uniform2f(off, 1, 1);
+  gl.drawArrays(gl.TRIANGLE_STRIP, 0, 3);
+  const w = gl.drawingBufferWidth, h = gl.drawingBufferHeight;
+  const px = new Uint8Array(w * h * 4);
+  gl.readPixels(0, 0, w, h, gl.RGBA, gl.UNSIGNED_BYTE, px);
+  // count small local extrema (|delta|<=3 to both horizontal neighbours, same
+  // sign) — the +-1-noise signature; a smooth/monotonic render has ~none.
+  let spikes = 0;
+  for (let y = 0; y < h; y++) {
+    for (let x = 1; x < w - 1; x++) {
+      for (let ch = 0; ch < 3; ch++) {
+        const i = (y * w + x) * 4 + ch; const val = px[i];
+        if (val === 0) continue;
+        const dl = val - px[i - 4], dr = val - px[i + 4];
+        if (dl * dr > 0 && Math.abs(dl) <= 3 && Math.abs(dr) <= 3) spikes++;
+      }
+    }
+  }
+  return { spikes: spikes, dims: w + 'x' + h };
+}"""
+
+
+@pytest.mark.e2e
+def test_webgl_readpixels_no_masking_signature(page):
+    """Stealth WebGL readPixels noise must be a coherent gamma remap (smooth),
+    not isolated +-1 flips. +-1 noise on the smooth gradient triangle produced
+    ~300+ 'spikes' and pixelscan flagged it as masking; the gamma remap leaves
+    the gradient smooth (~0 spikes). Regression guard for the gamma fix."""
+    res = _ev(page, _WEBGL_MASKING_PROBE)
+    assert "error" not in res, f"WebGL probe failed: {res}"
+    # genuine / gamma -> ~0; the rejected +-1 algorithm produced ~320.
+    assert res["spikes"] < 30, (
+        f"WebGL readPixels shows {res['spikes']} high-frequency noise spikes "
+        f"(pixelscan-maskable); the stealth noise must be a smooth gamma remap."
+    )

tests/test_headless.py

@@ -1,35 +1,39 @@
-"""Unit tests for the ``_headless`` virtual-display dispatcher.
+"""Unit tests for the ``_headless`` window-hider dispatcher.
 
-The dispatcher (``make_virtual_display``) is the only piece of
-``_headless`` we can exercise as a unit test on a single platform:
-``_WindowsVirtualDesktop`` actually creates a Win32 desktop on
-construction's later ``start()`` call, and ``_LinuxVirtualDisplay`` calls
-``Xvfb`` — both belong in integration/E2E coverage. The dispatcher's
-job is pure platform routing, which we patch via ``monkeypatch``.
+``make_virtual_display`` is pure platform routing:
+- Linux: a ``_LinuxVirtualDisplay`` (Xvfb) object the launcher start()s/stop()s.
+- Windows / macOS: ``None`` — the patched binary self-cloaks its chrome windows
+  via ``cloak_prefs()`` (injected by the launcher), so nothing host-side spawns.
+- Anything else: a clear ``RuntimeError`` naming the platform.
 
-Per scope: Windows-specific + platform-agnostic only. We still cover
-the Linux dispatch branch because instantiating ``_LinuxVirtualDisplay``
-does no I/O — Xvfb is only spawned in ``start()``, which we never call.
+``_LinuxVirtualDisplay`` construction does no I/O (Xvfb is only spawned in
+``start()``), so it's safe to exercise on any host.
 """
 from __future__ import annotations
 
-import sys
-
 import pytest
 
 import invisible_playwright._headless as headless
 from invisible_playwright._headless import (
+    CLOAK_PREFS,
     _LinuxVirtualDisplay,
-    _WindowsVirtualDesktop,
+    cloak_prefs,
     make_virtual_display,
 )
 
 
 @pytest.mark.unit
-def test_make_virtual_display_returns_windows_desktop_on_win32(monkeypatch):
+def test_make_virtual_display_returns_none_on_win32(monkeypatch):
+    """Windows hides via the in-binary cloak pref, not a host-side display."""
     monkeypatch.setattr(headless.sys, "platform", "win32")
-    vd = make_virtual_display()
-    assert isinstance(vd, _WindowsVirtualDesktop)
+    assert make_virtual_display() is None
+
+
+@pytest.mark.unit
+def test_make_virtual_display_returns_none_on_darwin(monkeypatch):
+    """macOS is now supported — it hides via the same in-binary cloak pref."""
+    monkeypatch.setattr(headless.sys, "platform", "darwin")
+    assert make_virtual_display() is None
 
 
 @pytest.mark.unit
@@ -37,8 +41,7 @@ def test_make_virtual_display_returns_linux_xvfb_on_linux(monkeypatch):
     """``__init__`` of ``_LinuxVirtualDisplay`` does no I/O — only ``start()``
     spawns Xvfb. Exercising the dispatcher here is safe on any host."""
     monkeypatch.setattr(headless.sys, "platform", "linux")
-    vd = make_virtual_display()
-    assert isinstance(vd, _LinuxVirtualDisplay)
+    assert isinstance(make_virtual_display(), _LinuxVirtualDisplay)
 
 
 @pytest.mark.unit
@@ -49,21 +52,10 @@ def test_make_virtual_display_accepts_linux_variants(monkeypatch):
     assert isinstance(make_virtual_display(), _LinuxVirtualDisplay)
 
 
-@pytest.mark.unit
-def test_make_virtual_display_raises_on_darwin(monkeypatch):
-    """macOS is unsupported — the dispatcher must raise with a clear
-    message rather than returning a no-op shim. ``InvisiblePlaywright``
-    relies on this to bail before launching Firefox on a system where
-    the patched binary doesn't exist."""
-    monkeypatch.setattr(headless.sys, "platform", "darwin")
-    with pytest.raises(RuntimeError, match="Windows and Linux only"):
-        make_virtual_display()
-
-
 @pytest.mark.unit
 def test_make_virtual_display_raises_on_unsupported_platform(monkeypatch):
     monkeypatch.setattr(headless.sys, "platform", "freebsd14")
-    with pytest.raises(RuntimeError, match="Windows and Linux only"):
+    with pytest.raises(RuntimeError, match="Windows, macOS and Linux"):
         make_virtual_display()
 
 
@@ -77,32 +69,13 @@ def test_make_virtual_display_error_mentions_offending_platform(monkeypatch):
 
 
 @pytest.mark.unit
-def test_windows_desktop_initial_state_is_clean():
-    """Construction must not allocate Win32 resources — only ``start()``
-    does. Allows users to instantiate ``InvisiblePlaywright`` without
-    pywin32 installed; the import error fires lazily when ``start()`` runs."""
-    vd = _WindowsVirtualDesktop()
-    assert vd._desktop is None
-    assert vd._original_handle == 0
-
-
-@pytest.mark.unit
-@pytest.mark.skipif(sys.platform != "win32", reason="exercises Win32 ctypes")
-def test_windows_desktop_stop_is_idempotent_without_start():
-    """``stop()`` after never calling ``start()`` must be a no-op, so
-    ``__exit__`` from a failed launch can call it unconditionally.
-
-    Skipped off Windows because ``stop()`` unconditionally resolves
-    ``ctypes.windll.user32`` at the top of the function — that symbol
-    only exists on Windows.  The early-return logic is safe because
-    callers only instantiate this class via ``make_virtual_display()``
-    which already routes on ``sys.platform == 'win32'``.
-    """
-    vd = _WindowsVirtualDesktop()
-    vd.stop()
-    vd.stop()
-    assert vd._desktop is None
-    assert vd._original_handle == 0
+def test_cloak_prefs_enables_cloak_and_disables_occlusion():
+    """The cloak prefs must turn on the in-binary cloak and turn OFF Windows
+    occlusion tracking (so a hidden window keeps painting). Returns a copy."""
+    p = cloak_prefs()
+    assert p["zoom.stealth.cloak_windows"] is True
+    assert p["widget.windows.window_occlusion_tracking.enabled"] is False
+    assert p == CLOAK_PREFS and p is not CLOAK_PREFS
 
 
 # ──────────────────────────────────────────────────────────────────────