feat: add primary display feature

app/Components/DisplayItem.xaml

@@ -26,6 +26,7 @@
         </Border.Style>
         <Grid>
             <TextBlock x:Name="xDeg" d:Text="90°" Foreground="White" Margin="8,4,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" />
+            <TextBlock x:Name="xPri" d:Text="[P]" Foreground="White" Margin="0,4,8,0" HorizontalAlignment="Right" VerticalAlignment="Top" />
             <TextBlock Text="{Binding DisplayNum}" d:Text="1" FontSize="24" FontWeight="SemiBold" Foreground="White" HorizontalAlignment="Center" VerticalAlignment="Center" Margin="0,0,0,5" />
             <TextBlock Grid.Row="1" Text="{Binding DisplayMode}" d:Text="1920 x 1080 @ 60 Hz" FontSize="12" Foreground="White" TextAlignment="Center" VerticalAlignment="Bottom" Margin="0,0,0,8" />
         </Grid>
@@ -52,6 +53,7 @@
             <Separator />
             <MenuItem Click="TakeScreenshot" Header="{DynamicResource t_take_screenshot}" IsEnabled="{Binding Active}" />
             <MenuItem Click="MirrorScreen" Header="{DynamicResource t_mirror_screen}" IsEnabled="{Binding Active}" />
+            <MenuItem Click="SetAsPrimary" Header="{DynamicResource t_set_primary}" IsEnabled="{Binding CanSetPrimary}" />
             <Separator />
             <MenuItem Click="RemoveDisplay" Header="{DynamicResource t_remove}" />
         </ContextMenu>

app/Components/DisplayItem.xaml.cs

@@ -11,6 +11,7 @@ namespace ParsecVDisplay.Components
     public partial class DisplayItem : UserControl
     {
         public bool Active { get; set; } = true;
+        public bool CanSetPrimary { get; set; } = true;
         public string DisplayNum { get; set; } = "1";
         public string DisplayName { get; set; } = "Display [1]";
         public string DisplayPath { get; set; } = "\\\\.\\DISPLAY1";
@@ -31,6 +32,7 @@ namespace ParsecVDisplay.Components
         {
             Display = display;
             Active = display.Active;
+            CanSetPrimary = display.Active && !display.Primary;
 
             DisplayNum = $"{display.Identifier}";
             DisplayName = $"Display [{display.Identifier}]";
@@ -40,6 +42,9 @@ namespace ParsecVDisplay.Components
                 ? Visibility.Hidden : Visibility.Visible;
             xDeg.Text = string.Format("{0}°", ((int)display.CurrentOrientation * 90));
 
+            xPri.Visibility = display.Primary ? Visibility.Visible : Visibility.Hidden;
+            xPri.Text = "[P]";
+
             if (display.Active)
             {
                 DisplayMode = display.CurrentMode.ToString();
@@ -236,5 +241,11 @@ namespace ParsecVDisplay.Components
         {
             Tray.Instance.Invoke(() => Tray.Instance.RemoveDisplay(Display.DisplayIndex));
         }
+
+        private void SetAsPrimary(object sender, RoutedEventArgs e)
+        {
+            if (Active && !Display.Primary)
+                Display.SetPrimary();
+        }
     }
 }

app/Display.cs

@@ -123,6 +123,7 @@ namespace ParsecVDisplay
         }
 
         public bool Active;
+        public bool Primary;
         public int Identifier;
         public int CloneOf;
         public int Address;
@@ -289,6 +290,60 @@ namespace ParsecVDisplay
             return Native.ChangeDisplaySettingsEx(null, IntPtr.Zero, IntPtr.Zero, 0, IntPtr.Zero) == 0;
         }
 
+        /// <summary>
+        /// Make this display the primary monitor. The desktop origin must land
+        /// on the new primary, so we shift every active display by this one's
+        /// current position before flagging it CDS_SET_PRIMARY, then commit
+        /// all changes atomically. Matches the vdd.c reference implementation.
+        /// </summary>
+        public bool SetPrimary()
+        {
+            if (string.IsNullOrEmpty(DeviceName))
+                return false;
+
+            // Capture our current position — every other display will be shifted
+            // by this offset so we end up at (0,0).
+            var target = new Native.DEVMODE();
+            target.dmSize = (short)Marshal.SizeOf(typeof(Native.DEVMODE));
+            if (!Native.EnumDisplaySettings(DeviceName, -1, ref target))
+                return false;
+
+            int offX = target.dmPositionX;
+            int offY = target.dmPositionY;
+
+            var dd = new Native.DISPLAY_DEVICE();
+            dd.cb = Marshal.SizeOf(typeof(Native.DISPLAY_DEVICE));
+
+            bool targetOk = false;
+            for (int i = 0; Native.EnumDisplayDevices(null, i, ref dd, 0); i++)
+            {
+                if ((dd.StateFlags & Native.DISPLAY_DEVICE_ACTIVE) == 0)
+                    continue;
+
+                var dm = new Native.DEVMODE();
+                dm.dmSize = (short)Marshal.SizeOf(typeof(Native.DEVMODE));
+                if (!Native.EnumDisplaySettings(dd.DeviceName, -1, ref dm))
+                    continue;
+
+                dm.dmPositionX -= offX;
+                dm.dmPositionY -= offY;
+                dm.dmFields = /*DM_POSITION*/ 0x20;
+
+                uint flags = /*CDS_UPDATEREGISTRY*/ 0x1 | /*CDS_NORESET*/ 0x10000000;
+                bool isTarget = string.Equals(dd.DeviceName, DeviceName, StringComparison.Ordinal);
+                if (isTarget)
+                    flags |= /*CDS_SET_PRIMARY*/ 0x10;
+
+                int rc = Native.ChangeDisplaySettingsEx(dd.DeviceName, ref dm, IntPtr.Zero, flags, IntPtr.Zero);
+                if (isTarget && rc == 0)
+                    targetOk = true;
+            }
+
+            // Commit pending positional changes + the primary flag together.
+            CommitChanges();
+            return targetOk;
+        }
+
         public void TakeScreenshot(string saveFile)
         {
             if (string.IsNullOrEmpty(DeviceName))
@@ -367,6 +422,8 @@ namespace ParsecVDisplay
                     var display = new Display
                     {
                         Active = (dd2.StateFlags & Native.DISPLAY_DEVICE_ACTIVE) != 0,
+                        // Primary is a per-adapter flag on the outer dd, not on the monitor
+                        Primary = (dd.StateFlags & Native.DISPLAY_DEVICE_PRIMARY_DEVICE) != 0,
                         Address = ParseDisplayAddress(dd2.DeviceID),
                         DeviceName = dd.DeviceName,
                         DisplayName = ParseDisplayCode(dd2.DeviceID),
@@ -507,6 +564,7 @@ namespace ParsecVDisplay
             public const uint EDD_GET_DEVICE_INTERFACE_NAME = 0x1;
             public const uint DISPLAY_DEVICE_ACTIVE = 0x1;
             public const uint DISPLAY_DEVICE_ATTACHED = 0x2;
+            public const uint DISPLAY_DEVICE_PRIMARY_DEVICE = 0x4;
 
             [DllImport("user32.dll")]
             [return: MarshalAs(UnmanagedType.Bool)]

app/Languages/en.xaml

@@ -23,6 +23,7 @@
         <system:String x:Key="t_portrait_flipped">Portrait (flipped)</system:String>
     <system:String x:Key="t_take_screenshot">Take screenshot</system:String>
     <system:String x:Key="t_mirror_screen">Mirror screen</system:String>
+    <system:String x:Key="t_set_primary">Set as primary</system:String>
     <system:String x:Key="t_remove">Remove</system:String>
     
     <!--Tray menu-->

app/Languages/vi.xaml

@@ -23,6 +23,7 @@
         <system:String x:Key="t_portrait_flipped">Dọc (lật ngược)</system:String>
     <system:String x:Key="t_take_screenshot">Chụp màn hình</system:String>
     <system:String x:Key="t_mirror_screen">Phản chiếu màn hình</system:String>
+    <system:String x:Key="t_set_primary">Đặt làm màn hình chính</system:String>
     <system:String x:Key="t_remove">Tháo bỏ</system:String>
     
     <!--Tray menu-->

app/Languages/zh.xaml

@@ -23,6 +23,7 @@
     <system:String x:Key="t_portrait_flipped">纵向 (翻转)</system:String>
     <system:String x:Key="t_take_screenshot">屏幕截图</system:String>
     <system:String x:Key="t_mirror_screen">镜像屏幕</system:String>
+    <system:String x:Key="t_set_primary">设为主显示器</system:String>
     <system:String x:Key="t_remove">移除</system:String>
 
     <!--Tray menu-->