feat: add device enumeration for PnP displays (#82)

- Legacy EnumDisplayDevices is bound to the calling thread, it's not working on RDP session.

app/Device.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using System.Text;
 
@@ -202,6 +203,58 @@ namespace ParsecVDisplay
             return Native.CM_Locate_DevNodeA(out devInst, deviceId, 0) == 0;
         }
 
+        public struct ClassDevice
+        {
+            public string InstanceId;
+            public uint DevInst;
+        }
+
+        /// <summary>
+        /// Enumerate present devices of a given setup class. Returns the device
+        /// instance ID (e.g. "DISPLAY\PSCCDD0\5&amp;abc&amp;UID256") and the
+        /// CM devnode handle for each. This goes through the PnP device tree
+        /// rather than GDI, so it is session-independent — useful when the app
+        /// runs inside an RDP session and EnumDisplayDevices is blind to the
+        /// console session's displays.
+        /// </summary>
+        public static List<ClassDevice> EnumerateClass(string classGuid)
+        {
+            var list = new List<ClassDevice>();
+
+            var guid = Guid.Parse(classGuid);
+            var devInfo = Native.SetupDiGetClassDevsA(ref guid, null, null, Native.DIGCF_PRESENT);
+            if (!devInfo.IsValidHandle())
+                return list;
+
+            try
+            {
+                var devInfoData = new Native.SP_DEVINFO_DATA();
+                devInfoData.cbSize = sizeof(Native.SP_DEVINFO_DATA);
+
+                const int bufSize = Native.MAX_DEVICE_ID_LEN;
+                byte* buf = stackalloc byte[bufSize];
+
+                for (uint i = 0; Native.SetupDiEnumDeviceInfo(devInfo, i, &devInfoData); i++)
+                {
+                    int required = 0;
+                    if (Native.SetupDiGetDeviceInstanceIdA(devInfo, &devInfoData, buf, bufSize, &required))
+                    {
+                        list.Add(new ClassDevice
+                        {
+                            InstanceId = Marshal.PtrToStringAnsi((IntPtr)buf),
+                            DevInst = devInfoData.DevInst,
+                        });
+                    }
+                }
+            }
+            finally
+            {
+                Native.SetupDiDestroyDeviceInfoList(devInfo);
+            }
+
+            return list;
+        }
+
         public static bool IsValidHandle(this IntPtr handle)
         {
             return handle != IntPtr.Zero
@@ -346,6 +399,15 @@ namespace ParsecVDisplay
                 int PropertyBufferSize,
                 int* RequiredSize);
 
+            [DllImport("setupapi.dll", SetLastError = true, EntryPoint = "SetupDiGetDeviceInstanceIdA", CharSet = CharSet.Ansi)]
+            [return: MarshalAs(UnmanagedType.Bool)]
+            public static extern bool SetupDiGetDeviceInstanceIdA(
+                IntPtr DeviceInfoSet,
+                SP_DEVINFO_DATA* DeviceInfoData,
+                byte* DeviceInstanceId,
+                int DeviceInstanceIdSize,
+                int* RequiredSize);
+
             [DllImport("setupapi.dll")]
             public static extern uint CM_Get_DevNode_Status(
                 uint* pulStatus,

app/Display.cs

@@ -291,14 +291,20 @@ namespace ParsecVDisplay
 
         public void TakeScreenshot(string saveFile)
         {
+            if (string.IsNullOrEmpty(DeviceName))
+                return;
+
             var devMode = new Native.DEVMODE();
             devMode.dmSize = (short)Marshal.SizeOf(typeof(Native.DEVMODE));
-            Native.EnumDisplaySettings(DeviceName, -1, ref devMode);
+            if (!Native.EnumDisplaySettings(DeviceName, -1, ref devMode))
+                return;
 
             int x = devMode.dmPositionX;
             int y = devMode.dmPositionY;
             int width = devMode.dmPelsWidth;
             int height = devMode.dmPelsHeight;
+            if (width <= 0 || height <= 0)
+                return;
 
             using (var bmp = new Bitmap(width, height))
             using (var gfx = Graphics.FromImage(bmp))
@@ -386,6 +392,35 @@ namespace ParsecVDisplay
                 }
             }
 
+            // Supplement with PnP-tree enumeration for Parsec monitors the GDI
+            // pass didn't see. Under RDP, EnumDisplayDevices is bound to the
+            // RDP session's window station and is blind to the console session's
+            // virtual displays — but SetupDi walks the kernel device tree and
+            // works regardless. These are added with Active=false (no GDI
+            // handle → can't ChangeDisplaySettings), so the UI shows them as
+            // [offline] but the count is correct.
+            const string CLASS_MONITOR_GUID = "{4d36e96e-e325-11ce-bfc1-08002be10318}";
+            foreach (var dev in Device.EnumerateClass(CLASS_MONITOR_GUID))
+            {
+                if (dev.InstanceId.IndexOf("PSCCDD0", StringComparison.OrdinalIgnoreCase) < 0)
+                    continue;
+                if (displayMap.ContainsKey(dev.InstanceId))
+                    continue;
+
+                var phantom = new Display
+                {
+                    Active = false,
+                    Address = ParseDisplayAddress(dev.InstanceId),
+                    DeviceName = string.Empty,
+                    DisplayName = "PSCCDD0",
+                };
+
+                if (Device.GetParentDeviceInstance(dev.DevInst, out uint parentInst, out phantom.AdapterInstance))
+                    phantom.AdapterArrival = Device.GetDeviceLastArrival(parentInst);
+
+                displayMap.Add(dev.InstanceId, phantom);
+            }
+
             var displays = displayMap.Values.ToList();
 
             // Sort displays by adapter arrival (older adapter → lower number),
@@ -395,7 +430,7 @@ namespace ParsecVDisplay
             displays.Sort((a, b) =>
             {
                 if (a.AdapterInstance == b.AdapterInstance)
-                    return a.DeviceName.CompareTo(b.DeviceName);
+                    return a.Address.CompareTo(b.Address);
                 return a.AdapterArrival.CompareTo(b.AdapterArrival);
             });
 

app/MirrorWindow.cs

@@ -48,6 +48,10 @@ namespace ParsecVDisplay
 
         public void MirrorScreen(string displayDevice)
         {
+            // Phantom (PnP-only) displays have no GDI handle; nothing to mirror.
+            if (string.IsNullOrEmpty(displayDevice))
+                return;
+
             if (!IsMirroring)
             {
                 IsMirroring = true;