Correctly handle DPI change and size in device independent units

src/WinUIEx/WindowEx.cs

@@ -26,6 +26,7 @@ public class WindowEx : Window
         private readonly ContentControl windowArea;
         private readonly WindowMessageMonitor mon;
         private readonly Microsoft.UI.Windowing.OverlappedPresenter overlappedPresenter;
+        private uint currentDpi;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="WindowEx"/> class.
@@ -34,6 +35,7 @@ public WindowEx()
         {
             overlappedPresenter = Microsoft.UI.Windowing.OverlappedPresenter.Create();
             AppWindow.SetPresenter(overlappedPresenter);
+            currentDpi = this.GetDpiForWindow();
 
             var rootContent = new Grid();
             rootContent.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(1, GridUnitType.Auto), MinHeight = 0 });
@@ -79,9 +81,28 @@ private unsafe void OnWindowMessage(object? sender, Messaging.WindowMessageEvent
                         rect2->ptMinTrackSize.y = Math.Max((int)MinHeight, 39);
                     }
                     break;
+                case WindowsMessages.WM_DPICHANGED:
+                    {
+                        var newDpi = this.GetDpiForWindow();
+                        if (newDpi != currentDpi)
+                        {
+                            var oldDpi = currentDpi;
+                            currentDpi = newDpi;
+                            OnDpiChanged(oldDpi, newDpi);
+                        }
+                        break;
+                    }
             }
         }
 
+        private void OnDpiChanged(uint oldDpi, uint newDpi)
+        {
+            var oldScale = oldDpi / 96f;
+            var currentSize = AppWindow.Size;
+            this.SetWindowSize((int)(currentSize.Width / oldScale), (int)(currentSize.Height / oldScale));
+            currentDpi = newDpi;
+        }
+
         private struct MINMAXINFO
         {
 #pragma warning disable CS0649
@@ -387,10 +408,10 @@ public Microsoft.UI.Windowing.AppWindowPresenterKind PresenterKind
         /// </summary>
         public double Width
         {
-            get { return AppWindow.Size.Width; }
+            get { return AppWindow.Size.Width / (currentDpi / 96d); }
             set
             {
-                this.SetWindowSize(value, AppWindow.Size.Height);
+                this.SetWindowSize(value, Height);
             }
         }
 
@@ -399,10 +420,10 @@ public double Width
         /// </summary>
         public double Height
         {
-            get { return AppWindow.Size.Height; }
+            get { return AppWindow.Size.Height / (currentDpi / 96d); }
             set
             {
-                this.SetWindowSize(AppWindow.Size.Width, value);
+                this.SetWindowSize(Width, value);
             }
         }
 

src/WinUIEx/WindowExtensions.cs

@@ -23,7 +23,7 @@ public static partial class WindowExtensions
         /// <remarks>
         /// <para><see href = "https://docs.microsoft.com/windows/win32/api//winuser/nf-winuser-getdpiforwindow">Learn more about this API from docs.microsoft.com</see>.</para>
         /// </remarks>
-        public static uint GetDpiForWindow(this Microsoft.UI.Xaml.Window window) => (uint?)(window.Content?.XamlRoot?.RasterizationScale * 96f) ?? HwndExtensions.GetDpiForWindow(window.GetWindowHandle());
+        public static uint GetDpiForWindow(this Microsoft.UI.Xaml.Window window) => HwndExtensions.GetDpiForWindow(window.GetWindowHandle());
 
         /// <summary>Brings the thread that created the specified window into the foreground and activates the window.</summary>
         /// <param name="window">
@@ -142,13 +142,15 @@ public static void CenterOnScreen(this Microsoft.UI.Xaml.Window window, double?
         /// Positions and resizes the window
         /// </summary>
         /// <param name="window">Window</param>
-        /// <param name="x">Left side of the window in device independent pixels</param>
-        /// <param name="y">Top  side of the window in device independent pixels</param>
+        /// <param name="x">Left side of the window</param>
+        /// <param name="y">Top side of the window</param>
         /// <param name="width">Width of the window in device independent pixels, or <c>null</c> if keeping the current size</param>
         /// <param name="height">Height of the window in device independent pixels, or <c>null</c> if keeping the current size</param>
         public static void MoveAndResize(this Microsoft.UI.Xaml.Window window, double x, double y, double width, double height)
-            => window.GetAppWindow().MoveAndResize(new Windows.Graphics.RectInt32((int)x, (int)y, (int)width, (int)height)); // TODO: Adjust for dpi
-            //=> HwndExtensions.SetWindowPositionAndSize(window.GetWindowHandle(), x, y, width, height);
+        {
+            var scale = HwndExtensions.GetDpiForWindow(window.GetWindowHandle()) / 96f;
+            window.GetAppWindow().MoveAndResize(new Windows.Graphics.RectInt32((int)x, (int)y, (int)(width * scale), (int)(height * scale)));
+        }
 
         /// <summary>
         /// Sets the width and height of the window in device-independent pixels.
@@ -157,7 +159,10 @@ public static void MoveAndResize(this Microsoft.UI.Xaml.Window window, double x,
         /// <param name="width">Width of the window in device-independent units.</param>
         /// <param name="height">Height of the window in device-independent units.</param>
         public static void SetWindowSize(this Microsoft.UI.Xaml.Window window, double width, double height)
-            => window.GetAppWindow().Resize(new Windows.Graphics.SizeInt32((int)width, (int)height));
+        {
+            var scale = HwndExtensions.GetDpiForWindow(window.GetWindowHandle()) / 96f;
+            window.GetAppWindow().Resize(new Windows.Graphics.SizeInt32((int)(width * scale), (int)(height * scale)));
+        }
 
         /// <summary>
         /// Sets the window presenter kind used.