Exclude virtual displays from the display clamp

headless='virtual' reaches launch_options as headless=False with
virtual_display set (async_api rewrites it), so the headful gate fired and
clamped the fingerprint to Xvfb's 1x1 stub. fix_screen_no_taskbar then drove
availHeight to -39 and validate_config rejected the launch outright.

pythonlib/camoufox/utils.py

@@ -705,11 +705,11 @@ def launch_options(
     if not _user_set_navigator:
         fix_navigator_arch(config, target_os)
     if not _user_set_screen_window:
-        # Headful only: this bound exists so the real window fits the monitor.
-        # headless has no window to overflow, and headless='virtual' runs a 1x1
-        # Xvfb (see virtdisplay.py) that would otherwise shrink the whole
-        # fingerprint to 1x1.
-        if headless is False and screen_cons:
+        # Headful on a real monitor only: this bound exists so the window fits
+        # the screen it is drawn on. headless has no window to overflow, and
+        # headless='virtual' reaches here as headless=False (see async_api) with
+        # a 1x1 Xvfb (virtdisplay.py) that is not a real screen.
+        if headless is False and not virtual_display and screen_cons:
             clamp_screen_to_display(config, screen_cons.max_width, screen_cons.max_height)
         fix_screen_no_taskbar(config, target_os)
         clamp_window_dimensions(config)

pythonlib/tests/test_launch_geometry.py

@@ -0,0 +1,70 @@
+"""
+Launch-level guards for the display clamp in launch_options().
+
+Run with:
+    cd pythonlib && python -m pytest tests/test_launch_geometry.py -v
+"""
+
+import os
+import sys
+from contextlib import contextmanager
+from unittest import mock
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+import orjson  # noqa: E402
+from browserforge.fingerprints import Screen  # noqa: E402
+
+from camoufox import utils  # noqa: E402
+
+# What get_screen_cons() reports under the Xvfb that headless='virtual' starts:
+# virtdisplay.py sizes it "1x1x24", and launch_options mutates os.environ's
+# DISPLAY, so the parent process enumerates that stub as its only monitor.
+XVFB_STUB = Screen(max_width=1, max_height=1)
+
+
+@contextmanager
+def host(screen_cons):
+    """Run launch_options() against a stubbed host, without touching the disk."""
+    with mock.patch.object(utils, "get_screen_cons", lambda headless: screen_cons), (
+        mock.patch.object(utils, "installed_verstr", lambda: "150.0.2")
+    ), mock.patch.object(utils, "launch_path", lambda **kwargs: "/nonexistent/camoufox"):
+        yield
+
+
+def config_of(options):
+    """Reassemble the chunked CAMOU_CONFIG_<n> env vars into a dict."""
+    env = options["env"]
+    chunks = sorted(
+        (int(k.rsplit("_", 1)[1]), v) for k, v in env.items() if k.startswith("CAMOU_CONFIG_")
+    )
+    return orjson.loads("".join(chunk for _, chunk in chunks))
+
+
+def launch(**kwargs):
+    kwargs.setdefault("os", "windows")
+    kwargs.setdefault("i_know_what_im_doing", True)
+    return config_of(utils.launch_options(**kwargs))
+
+
+class TestVirtualDisplayIsNotAScreen:
+    """headless='virtual' arrives here as headless=False (async_api rewrites it),
+    so the headful gate alone would clamp the fingerprint to the 1x1 Xvfb."""
+
+    def test_fingerprint_is_not_shrunk_to_the_xvfb(self):
+        with host(XVFB_STUB):
+            config = launch(headless=False, virtual_display=":99")
+
+        assert config["screen.width"] > 1
+        assert config["screen.height"] > 1
+        assert config["window.outerWidth"] > 1
+
+    def test_screen_dimensions_stay_valid(self):
+        """Clamping to 1x1 drives availHeight negative once fix_screen_no_taskbar
+        subtracts the taskbar, which validate_config rejects as a uint."""
+        with host(XVFB_STUB):
+            config = launch(headless=False, virtual_display=":99")
+
+        for key, value in config.items():
+            if key.startswith("screen.") or key.startswith("window.outer"):
+                assert value >= 0, f"{key} is negative: {value}"