diff --git a/win/dpi_aware.py b/win/dpi_aware.py
new file mode 100644
index 0000000..a3daac4
--- /dev/null
+++ b/win/dpi_aware.py
@@ -0,0 +1,61 @@
+#!/usr/bin/env python3
+"""Per-Monitor DPI Awareness V2 bootstrap for the OpenSpan Windows side.
+
+Must run in EVERY OpenSpan process (control GUI, setup window, input portal)
+before any Tk window or Win32 monitor/hook API is touched. The portal's edge
+geometry, the monitor rectangles it / the GUI enumerate (written to
+openspan_config.json) and the low-level mouse-hook coordinates all have to
+live in ONE physical-pixel coordinate space. On this machine that means the
+three real sizes 1600x2560 / 1920x1080 / 2560x1600 -- not the virtualized
+logical sizes (800x1280 / 1536x864 / 1280x800) a DPI-unaware or mixed-DPI
+process reports, which made the portal fire before the mouse reached the
+real screen edge.
+
+Pure standard library (ctypes), matching the rest of the repo.
+"""
+
+import ctypes
+
+
+def _current():
+    """Query the process's current DPI awareness via shcore. Returns a
+    label ('unaware'/'system'/'pm') or None when the API is unavailable."""
+    try:
+        val = ctypes.c_int(-1)
+        ctypes.windll.shcore.GetProcessDpiAwareness(
+            ctypes.c_void_p(0), ctypes.byref(val))
+    except (AttributeError, OSError):
+        return None
+    return {0: "unaware", 1: "system", 2: "pm"}.get(val.value)
+
+
+def ensure():
+    """Declare Per-Monitor DPI Awareness V2 if not already set.
+
+    Prefers SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_
+    AWARE_V2 = -4) (Windows 10 1703+). If the process is already aware --
+    a manifest or an earlier setter -- the call fails with
+    ERROR_ACCESS_DENIED and the existing awareness wins, which is fine
+    because _current() reports what is actually in force.
+
+    Fallbacks in order: shcore SetProcessDpiAwareness(PROCESS_PER_MONITOR_
+    DPI_AWARE), user32 SetProcessDPIAware (system-aware). Returns the label
+    in force afterwards.
+    """
+    try:
+        ctx = ctypes.c_void_p(-4)
+        if ctypes.windll.user32.SetProcessDpiAwarenessContext(ctx):
+            return _current() or "v2"
+    except (AttributeError, OSError):
+        pass
+    try:
+        if ctypes.windll.shcore.SetProcessDpiAwareness(2) == 0:
+            return _current() or "pm"
+    except (AttributeError, OSError):
+        pass
+    try:
+        if ctypes.windll.user32.SetProcessDPIAware():
+            return "system"
+    except (AttributeError, OSError):
+        pass
+    return _current() or "none"
diff --git a/win/openspan_launcher.py b/win/openspan_launcher.py
index ad3016b..8390271 100644
--- a/win/openspan_launcher.py
+++ b/win/openspan_launcher.py
@@ -67,6 +67,8 @@ def _traytest():
 
 if __name__ == "__main__":
     _ensure_std_streams()
+    import dpi_aware  # noqa: E402
+    dpi_aware.ensure()  # physical-pixel space for every role (see dpi_aware.py)
     arg = sys.argv[1] if len(sys.argv) > 1 else ""
     if arg == "--portal":
         del sys.argv[1]
diff --git a/win/openspan_portal.py b/win/openspan_portal.py
index d1a12d7..c2a8c9f 100644
--- a/win/openspan_portal.py
+++ b/win/openspan_portal.py
@@ -24,6 +24,13 @@
 import time
 import queue
 
+# Physical-pixel coordinate space BEFORE any Win32 monitor / hook call:
+# portal edges (openspan_config.json), hook coords (ms.pt) and monitor
+# rectangles all have to be the SAME space, or the portal fires early on
+# mixed-DPI desktops.
+import dpi_aware  # noqa: E402
+dpi_aware.ensure()  # noqa: E402
+
 user32 = ctypes.windll.user32
 kernel32 = ctypes.windll.kernel32
 
diff --git a/win/openspan_setup.py b/win/openspan_setup.py
index f3dabc5..7113843 100644
--- a/win/openspan_setup.py
+++ b/win/openspan_setup.py
@@ -18,6 +18,11 @@
 import tkinter as tk
 from tkinter import ttk, messagebox
 
+# Same physical-pixel space for the GUI, the setup window and the portal:
+# must be declared before any Tk window / GetMonitorInfo call.
+import dpi_aware  # noqa: E402
+dpi_aware.ensure()  # noqa: E402
+
 if getattr(sys, "frozen", False):  # frozen exe: data lives at the exe
     CONFIG_PATH = os.path.join(
         os.path.dirname(os.path.abspath(sys.executable)),