test(prefs, headless): add 16 Linux-specific tests for Phase 6 + 10

Cover the Linux branches in prefs.py that previously had no tests
(font metrics, GPU spoofing, MSAA from profile, canvas noise mask
per renderer, WebGL extension preservation, Xvfb workarounds,
virtual_display no-op) and add construction smoke tests for
_LinuxVirtualDisplay. Also fix two host-platform-dependent tests
so the suite stays green on both Windows and Linux.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_headless.py

@@ -13,6 +13,8 @@ does no I/O — Xvfb is only spawned in ``start()``, which we never call.
 """
 from __future__ import annotations
 
+import sys
+
 import pytest
 
 import invisible_playwright._headless as headless
@@ -85,11 +87,65 @@ def test_windows_desktop_initial_state_is_clean():
 
 
 @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."""
+    ``__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
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  _LinuxVirtualDisplay — construction-only smoke tests. ``start()`` is
+#  E2E because it spawns Xvfb; ``stop()`` is safe to call when no Xvfb
+#  was ever started, so we exercise that path explicitly.
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_linux_virtual_display_initial_state_is_clean():
+    """Construction must not spawn Xvfb or mutate the environment — only
+    ``start()`` does. Mirrors the Windows construction-state test."""
+    vd = _LinuxVirtualDisplay()
+    assert vd._proc is None
+    assert vd._display is None
+    assert vd._saved_env == {}
+
+
+@pytest.mark.unit
+def test_linux_virtual_display_geometry_default():
+    """Default geometry is 1920x1080x24 — matches the profile sampler's
+    default screen and avoids the Xvfb default of 1280x1024 which the
+    fingerprint pipeline never produces."""
+    vd = _LinuxVirtualDisplay()
+    assert vd._geometry == "1920x1080x24"
+
+
+@pytest.mark.unit
+def test_linux_virtual_display_custom_geometry():
+    """Caller-supplied width/height feed straight into the Xvfb geometry
+    spec; the depth is always 24 (Firefox/ANGLE assume true-color)."""
+    vd = _LinuxVirtualDisplay(width=2560, height=1440)
+    assert vd._geometry == "2560x1440x24"
+
+
+@pytest.mark.unit
+def test_linux_virtual_display_stop_without_start_is_safe():
+    """``stop()`` before ``start()`` must be a no-op — supports the
+    ``__exit__`` path on a launcher that failed before Xvfb was spawned.
+    Verifies no AttributeError on env restore (saved_env is empty)."""
+    vd = _LinuxVirtualDisplay()
+    vd.stop()
+    vd.stop()
+    assert vd._proc is None
+    assert vd._display is None

tests/test_prefs.py

@@ -5,6 +5,7 @@ import pytest
 
 from invisible_playwright._fpforge import generate_profile
 from invisible_playwright.prefs import (
+    _LINUX_GENERIC_FONT_FACTORS,
     _accept_language,
     _font_metrics_for_platform,
     _WIN_LIGHT_COLORS,
@@ -13,8 +14,9 @@ from invisible_playwright.prefs import (
 
 
 @pytest.mark.unit
-def test_translate_includes_gpu_renderer_windows():
+def test_translate_includes_gpu_renderer_windows(monkeypatch):
     """On Windows, renderer/vendor are cleared so ANGLE reports native hardware."""
+    monkeypatch.setattr(sys, "platform", "win32")
     p = generate_profile(seed=42)
     prefs = translate_profile_to_prefs(p)
     assert prefs["zoom.stealth.webgl.renderer"] == ""
@@ -356,3 +358,158 @@ def test_lan_ip_seed_zero_has_no_zero_octets():
     assert octets[1] == "168"
     assert int(octets[2]) >= 1
     assert int(octets[3]) >= 1
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  Linux-specific tests — exercise the branches that only fire when
+#  ``sys.platform.startswith("linux")``. Patched via ``monkeypatch`` so
+#  these run on any host CI environment.
+# ──────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+def test_font_metrics_linux_prepends_generic_factors(monkeypatch):
+    # FM1: Linux prepends the GTK/DejaVu compensation block to the
+    # per-font metrics string sampled from the profile.
+    monkeypatch.setattr(sys, "platform", "linux")
+    out = _font_metrics_for_platform("Arial|1.0,Verdana|0.9,")
+    assert out.startswith(_LINUX_GENERIC_FONT_FACTORS)
+    assert out.endswith("Arial|1.0,Verdana|0.9,")
+
+
+@pytest.mark.unit
+def test_font_metrics_linux_empty_input_returns_empty(monkeypatch):
+    # FM1b: even on Linux, empty profile metrics short-circuits before
+    # the prepend so we never emit a metrics pref containing only the
+    # generic block (which would surface as a tampering signal).
+    monkeypatch.setattr(sys, "platform", "linux")
+    assert _font_metrics_for_platform("") == ""
+
+
+@pytest.mark.unit
+def test_font_metrics_linux2_variant_uses_linux_branch(monkeypatch):
+    # FM1c: ``sys.platform`` can be ``linux2`` on older Pythons / odd
+    # WSL builds. ``startswith("linux")`` accepts both.
+    monkeypatch.setattr(sys, "platform", "linux2")
+    out = _font_metrics_for_platform("Verdana|0.9,")
+    assert out.startswith(_LINUX_GENERIC_FONT_FACTORS)
+
+
+@pytest.mark.unit
+def test_gpu_renderer_set_from_profile_on_linux(monkeypatch):
+    # PG1: on Linux we spoof to the profile's Windows-ANGLE renderer
+    # string so cross-platform sessions present a consistent Windows GPU.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.renderer"] == p.gpu.renderer
+    assert prefs["zoom.stealth.webgl.vendor"] == p.gpu.vendor
+    assert prefs["zoom.stealth.webgl.renderer"]  # non-empty
+
+
+@pytest.mark.unit
+def test_msaa_from_profile_on_linux(monkeypatch):
+    # PG3: on Linux, MSAA comes from the profile's sampled value rather
+    # than being pinned to 4 (which is the Windows ANGLE default).
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42, pin={"webgl.msaa_samples": 8})
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.msaa"] == 8
+    assert prefs["webgl.msaa-samples"] == 8
+    assert prefs["webgl.msaa-force"] is True
+
+
+@pytest.mark.unit
+def test_msaa_zero_disables_force_on_linux(monkeypatch):
+    # PG3b: MSAA=0 means "no MSAA" so ``webgl.msaa-force`` must be False.
+    # Verifies the ``> 0`` guard on the force flag.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42, pin={"webgl.msaa_samples": 0})
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.msaa"] == 0
+    assert prefs["webgl.msaa-force"] is False
+
+
+@pytest.mark.unit
+def test_canvas_noise_mask_intel_on_linux(monkeypatch):
+    # CN1: Intel renderer → 1/16 noise (mask=15). Pinning the renderer
+    # exercises the live ``_renderer_lo`` branch on Linux (where the
+    # value is read from the profile rather than hardcoded as on Windows).
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(
+        seed=42,
+        pin={
+            "gpu.renderer": "ANGLE (Intel, Intel(R) UHD Graphics 630 Direct3D11 vs_5_0 ps_5_0, D3D11)",
+            "gpu.vendor": "Google Inc. (Intel)",
+        },
+    )
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.canvas.noise_skip_mask"] == 15
+
+
+@pytest.mark.unit
+def test_canvas_noise_mask_nvidia_on_linux(monkeypatch):
+    # CN2: NVIDIA/AMD renderer → 1/8 noise (mask=7). The "intel" substring
+    # check must NOT match here.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(
+        seed=42,
+        pin={
+            "gpu.renderer": "ANGLE (NVIDIA, NVIDIA GeForce RTX 4090 Direct3D11 vs_5_0 ps_5_0, D3D11)",
+            "gpu.vendor": "Google Inc. (NVIDIA)",
+        },
+    )
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.canvas.noise_skip_mask"] == 7
+
+
+@pytest.mark.unit
+def test_webgl_extensions_preserved_on_linux(monkeypatch):
+    # WE1: on Linux the curated WebGL1/2 extension lists from _BASELINE
+    # remain in the prefs dict so the patched binary publishes them
+    # instead of native Mesa's set.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["zoom.stealth.webgl.extensions"]
+    assert prefs["zoom.stealth.webgl2.extensions"]
+    # Spot-check a canonical Windows ANGLE extension is in the list.
+    assert "ANGLE_instanced_arrays" in prefs["zoom.stealth.webgl.extensions"]
+    assert "OVR_multiview2" in prefs["zoom.stealth.webgl2.extensions"]
+
+
+@pytest.mark.unit
+def test_xvfb_workarounds_applied_on_linux(monkeypatch):
+    # XW1: Linux Firefox under Xvfb can't run WebRender, so we force the
+    # software path. These are added via ``setdefault`` so callers can
+    # still override them via ``extra_prefs``.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p)
+    assert prefs["gfx.webrender.all"] is False
+    assert prefs["gfx.webrender.force-disabled"] is True
+    assert prefs["webgl.force-enabled"] is True
+
+
+@pytest.mark.unit
+def test_xvfb_workarounds_caller_can_override(monkeypatch):
+    # XW1b: the workarounds are added with ``setdefault``, so a user-
+    # supplied ``extra_prefs`` value wins. Verifies the override path
+    # doesn't get clobbered by the platform branch.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(
+        p, extra_prefs={"webgl.force-enabled": False}
+    )
+    assert prefs["webgl.force-enabled"] is False
+
+
+@pytest.mark.unit
+def test_virtual_display_no_op_on_linux(monkeypatch):
+    # VD3: ``virtual_display`` is a Windows-only concept (CreateDesktop
+    # alt-desktop GPU sandbox workaround). Even when True, Linux must
+    # not pick up ``security.sandbox.gpu.level``.
+    monkeypatch.setattr(sys, "platform", "linux")
+    p = generate_profile(seed=42)
+    prefs = translate_profile_to_prefs(p, virtual_display=True)
+    assert "security.sandbox.gpu.level" not in prefs