Probe the host monitor in CSS pixels

screeninfo makes the process per-monitor DPI aware, so it reports physical pixels, while Firefox lays windows out in CSS pixels. At 150% Windows scaling a 1920x1080 panel is 1280x720 CSS px, so bounding the fingerprint by the physical size lets the window open 1.5x larger than the screen. Refs #425
2026-09-10 00:00:42 +00:00 · 2026-07-28 00:29:03 -06:00
parent fbafbcf9f0
commit 22c6ffbdda
3 changed files with 201 additions and 11 deletions
@@ -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