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