diff --git a/pythonlib/camoufox/display.py b/pythonlib/camoufox/display.py
new file mode 100644
index 0000000..a4243af
--- /dev/null
+++ b/pythonlib/camoufox/display.py
@@ -0,0 +1,101 @@
+"""
+Host display geometry, in the units Firefox lays its windows out in.
+
+Firefox sizes windows in **CSS pixels**, but `screeninfo` marks the process
+per-monitor DPI aware and therefore reports **physical** pixels. Where Windows
+display scaling is enabled the two differ by the scale factor: a 1920x1080 panel
+at 150% is only 1280x720 CSS px. Deriving a window size from the physical
+numbers overshoots the screen by that factor, so the window opens partly
+off-screen (daijro/camoufox#425).
+
+macOS (`NSScreen.frame`) and X11 (xrandr) already report CSS pixels, so scaling
+only ever applies on Windows.
+"""
+
+from typing import Any, NamedTuple, Optional
+
+from screeninfo import get_monitors
+
+from .pkgman import OS_NAME
+
+# Windows expresses DPI relative to this baseline: 144 DPI == 150% scaling.
+_WINDOWS_BASE_DPI = 96
+
+# shcore.h / winuser.h constants
+_MDT_EFFECTIVE_DPI = 0
+_MONITOR_DEFAULTTONEAREST = 2
+
+
+class DisplaySize(NamedTuple):
+    """Size of a monitor in CSS pixels."""
+
+    width: int
+    height: int
+
+
+def largest_display() -> Optional[DisplaySize]:
+    """
+    Size of the roomiest attached monitor in CSS pixels, or None when the
+    display cannot be probed (no monitors, or enumeration failed).
+    """
+    try:
+        monitors = get_monitors()
+    except Exception:
+        return None
+    if not monitors:
+        return None
+
+    monitor = max(monitors, key=lambda m: m.width * m.height)
+    scale = _scale_factor(monitor)
+    return DisplaySize(
+        width=max(1, int(monitor.width / scale)),
+        height=max(1, int(monitor.height / scale)),
+    )
+
+
+def _scale_factor(monitor: Any) -> float:
+    """
+    Physical pixels per CSS pixel on `monitor`. Always 1.0 outside Windows.
+    """
+    if OS_NAME != 'win':
+        return 1.0
+    try:
+        dpi = _windows_monitor_dpi(monitor)
+    except Exception:
+        return 1.0  # Pre-Windows 8.1, or the shcore call is unavailable
+    return dpi / _WINDOWS_BASE_DPI if dpi > 0 else 1.0
+
+
+def _windows_monitor_dpi(monitor: Any) -> int:
+    """
+    Effective DPI of `monitor`, via shcore!GetDpiForMonitor (Windows 8.1+).
+
+    Private WinDLL handles are used rather than the process-wide `ctypes.windll`
+    cache so that annotating the prototypes cannot affect other libraries.
+    """
+    import ctypes
+    from ctypes import wintypes
+
+    user32 = ctypes.WinDLL('user32')  # type: ignore[attr-defined]
+    user32.MonitorFromPoint.argtypes = (wintypes.POINT, wintypes.DWORD)
+    user32.MonitorFromPoint.restype = wintypes.HANDLE
+    handle = user32.MonitorFromPoint(
+        wintypes.POINT(int(monitor.x), int(monitor.y)), _MONITOR_DEFAULTTONEAREST
+    )
+
+    shcore = ctypes.WinDLL('shcore')  # type: ignore[attr-defined]
+    shcore.GetDpiForMonitor.argtypes = (
+        wintypes.HANDLE,
+        ctypes.c_int,
+        ctypes.POINTER(wintypes.UINT),
+        ctypes.POINTER(wintypes.UINT),
+    )
+    shcore.GetDpiForMonitor.restype = ctypes.c_long  # HRESULT
+    dpi_x, dpi_y = wintypes.UINT(), wintypes.UINT()
+
+    hresult = shcore.GetDpiForMonitor(
+        handle, _MDT_EFFECTIVE_DPI, ctypes.byref(dpi_x), ctypes.byref(dpi_y)
+    )
+    if hresult != 0:
+        raise OSError(f'GetDpiForMonitor failed (0x{hresult & 0xFFFFFFFF:08X})')
+    return dpi_x.value
diff --git a/pythonlib/camoufox/utils.py b/pythonlib/camoufox/utils.py
index 88137bf..2f5f9ea 100644
--- a/pythonlib/camoufox/utils.py
+++ b/pythonlib/camoufox/utils.py
@@ -11,11 +11,11 @@ from typing import Any, Dict, List, Literal, Optional, Tuple, Union
 import numpy as np
 import orjson
 from browserforge.fingerprints import Fingerprint, Screen
-from screeninfo import get_monitors
 from typing_extensions import TypeAlias
 from ua_parser import user_agent_parser
 
 from .addons import DefaultAddons, add_default_addons, confirm_paths
+from .display import largest_display
 from .exceptions import (
     InvalidOS,
     InvalidPropertyType,
@@ -221,19 +221,16 @@ def determine_ua_os(user_agent: str) -> Literal['mac', 'win', 'lin']:
 def get_screen_cons(headless: Optional[bool] = None) -> Optional[Screen]:
     """
     Determines a sane viewport size for Camoufox if being ran in headful mode.
+
+    Bounds are CSS pixels, the unit Firefox lays its windows out in -- see
+    camoufox.display for why that differs from the monitor's physical size.
     """
     if headless is False:
         return None  # Skip if headless
-    try:
-        monitors = get_monitors()
-    except Exception:
-        return None  # Skip if there's an error getting the monitors
-    if not monitors:
-        return None  # Skip if there are no monitors
-
-    # Use the dimensions from the monitor with greatest screen real estate
-    monitor = max(monitors, key=lambda m: m.width * m.height)
-    return Screen(max_width=monitor.width, max_height=monitor.height)
+    display = largest_display()
+    if display is None:
+        return None  # Skip if the display can't be probed
+    return Screen(max_width=display.width, max_height=display.height)
 
 
 def update_fonts(config: Dict[str, Any], target_os: str) -> None:
diff --git a/pythonlib/tests/test_display.py b/pythonlib/tests/test_display.py
new file mode 100644
index 0000000..0729b87
--- /dev/null
+++ b/pythonlib/tests/test_display.py
@@ -0,0 +1,92 @@
+"""
+Tests for camoufox.display -- probing the host monitor in CSS pixels.
+
+Guards daijro/camoufox#425: with Windows display scaling enabled, screeninfo
+reports physical pixels while Firefox lays windows out in CSS pixels, so the
+browser window opened larger than the screen.
+
+Run with:
+    cd pythonlib && python -m pytest tests/test_display.py -v
+"""
+
+import os
+import sys
+from types import SimpleNamespace
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+import pytest  # noqa: E402
+
+from camoufox import display  # noqa: E402
+
+
+def monitor(width, height, x=0, y=0):
+    return SimpleNamespace(width=width, height=height, x=x, y=y)
+
+
+@pytest.fixture
+def monitors(monkeypatch):
+    """Stub out screeninfo with an explicit monitor list."""
+
+    def _set(*found):
+        monkeypatch.setattr(display, "get_monitors", lambda: list(found))
+
+    return _set
+
+
+class TestLargestDisplay:
+    def test_reports_unscaled_display_verbatim(self, monkeypatch, monitors):
+        monkeypatch.setattr(display, "OS_NAME", "lin")
+        monitors(monitor(1920, 1080))
+        assert display.largest_display() == (1920, 1080)
+
+    @pytest.mark.parametrize(
+        ("dpi", "expected"),
+        [(96, (1920, 1080)), (120, (1536, 864)), (144, (1280, 720)), (192, (960, 540))],
+    )
+    def test_windows_scaling_converts_to_css_pixels(
+        self, monkeypatch, monitors, dpi, expected
+    ):
+        monkeypatch.setattr(display, "OS_NAME", "win")
+        monkeypatch.setattr(display, "_windows_monitor_dpi", lambda m: dpi)
+        monitors(monitor(1920, 1080))
+        assert display.largest_display() == expected
+
+    def test_scaling_is_windows_only(self, monkeypatch, monitors):
+        """macOS and X11 already report CSS pixels; never rescale them."""
+        monkeypatch.setattr(display, "_windows_monitor_dpi", lambda m: 192)
+        monitors(monitor(1920, 1080))
+        for os_name in ("mac", "lin"):
+            monkeypatch.setattr(display, "OS_NAME", os_name)
+            assert display.largest_display() == (1920, 1080)
+
+    def test_falls_back_to_1x_when_dpi_lookup_fails(self, monkeypatch, monitors):
+        def unavailable(_):
+            raise OSError("GetDpiForMonitor failed")
+
+        monkeypatch.setattr(display, "OS_NAME", "win")
+        monkeypatch.setattr(display, "_windows_monitor_dpi", unavailable)
+        monitors(monitor(1920, 1080))
+        assert display.largest_display() == (1920, 1080)
+
+    def test_falls_back_to_1x_on_nonsense_dpi(self, monkeypatch, monitors):
+        monkeypatch.setattr(display, "OS_NAME", "win")
+        monkeypatch.setattr(display, "_windows_monitor_dpi", lambda m: 0)
+        monitors(monitor(1920, 1080))
+        assert display.largest_display() == (1920, 1080)
+
+    def test_picks_the_roomiest_monitor(self, monkeypatch, monitors):
+        monkeypatch.setattr(display, "OS_NAME", "lin")
+        monitors(monitor(1280, 720), monitor(2560, 1440), monitor(1920, 1080))
+        assert display.largest_display() == (2560, 1440)
+
+    def test_none_when_no_monitors(self, monitors):
+        monitors()
+        assert display.largest_display() is None
+
+    def test_none_when_enumeration_raises(self, monkeypatch):
+        def boom():
+            raise RuntimeError("no display")
+
+        monkeypatch.setattr(display, "get_monitors", boom)
+        assert display.largest_display() is None