Rewrite WindowImpl.Resize to use SetWindowPlacement

Author: TomEdwardsEnscape

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -105,18 +105,58 @@ public enum SizeCommand
 
         public enum ShowWindowCommand
         {
+            /// <summary>
+            /// Hides the window and activates another window.
+            /// </summary>
             Hide = 0,
+            /// <summary>
+            /// Activates and displays a window. If the window is minimized, maximized, or arranged, the system restores it to its original 
+            /// size and position. An application should specify this flag when displaying the window for the first time.
+            /// </summary>
             Normal = 1,
+            /// <summary>
+            /// Activates the window and displays it as a minimized window.
+            /// </summary>
             ShowMinimized = 2,
+            /// <summary>
+            /// Activates the window and displays it as a maximized window.
+            /// </summary>
             Maximize = 3,
-            ShowMaximized = 3,
+            /// <inheritdoc cref="Maximize"/>
+            ShowMaximized = Maximize,
+            /// <summary>
+            /// Displays a window in its most recent size and position. This value is similar to <see cref="Normal"/>, except that the window is not activated.
+            /// </summary>
             ShowNoActivate = 4,
+            /// <summary>
+            /// Activates the window and displays it in its current size and position.
+            /// </summary>
             Show = 5,
+            /// <summary>
+            /// Minimizes the specified window and activates the next top-level window in the Z order.
+            /// </summary>
             Minimize = 6,
+            /// <summary>
+            /// Displays the window as a minimized window. This value is similar to <see cref="ShowMinimized"/>, except the window is not activated.
+            /// </summary>
             ShowMinNoActive = 7,
+            /// <summary>
+            /// Displays the window in its current size and position. This value is similar to <see cref="Show"/>, except that the window is not activated.
+            /// </summary>
             ShowNA = 8,
+            /// <summary>
+            /// Activates and displays the window. If the window is minimized, maximized, or arranged, the system restores it to its original size and position. 
+            /// An application should specify this flag when restoring a minimized window.
+            /// </summary>
             Restore = 9,
+            /// <summary>
+            /// Sets the show state based on the <see cref="ShowWindowCommand"/> value specified in the STARTUPINFO structure passed to the CreateProcess function 
+            /// by the program that started the application.
+            /// </summary>
             ShowDefault = 10,
+            /// <summary>
+            /// Minimizes a window, even if the thread that owns the window is not responding. This flag should only be used when minimizing windows from a different thread.
+            /// </summary>
             ForceMinimize = 11
         }
 
@@ -1160,6 +1200,9 @@ public static extern int SetDIBitsToDevice(IntPtr hdc, int XDest, int YDest,
         [DllImport("user32.dll", SetLastError = true)]
         public static extern bool AdjustWindowRectEx(ref RECT lpRect, uint dwStyle, bool bMenu, uint dwExStyle);
 
+        [DllImport("user32.dll", SetLastError = true)]
+        public static extern bool AdjustWindowRectExForDpi(ref RECT lpRect, WindowStyles dwStyle, bool bMenu, WindowStyles dwExStyle, uint dpi);
+
         [DllImport("user32.dll")]
         public static extern IntPtr BeginPaint(IntPtr hwnd, out PAINTSTRUCT lpPaint);
 
@@ -1287,7 +1330,7 @@ public static IntPtr SetWindowLongPtr(IntPtr hWnd, int nIndex, IntPtr handle)
         public static extern bool EnableMenuItem(IntPtr hMenu, uint uIDEnableItem, uint uEnable);
 
         [DllImport("user32.dll", SetLastError = true)]
-        public static extern bool GetWindowPlacement(IntPtr hWnd, ref WINDOWPLACEMENT lpwndpl);
+        public static extern bool GetWindowPlacement(IntPtr hWnd, out WINDOWPLACEMENT lpwndpl);
 
         [DllImport("user32.dll")]
         public static extern bool GetWindowRect(IntPtr hwnd, out RECT lpRect);
@@ -1374,6 +1417,8 @@ public static extern IntPtr CreateIconFromResourceEx(byte* pbIconBits, uint cbIc
         [DllImport("user32.dll")]
         public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int x, int y, int cx, int cy, SetWindowPosFlags uFlags);
         [DllImport("user32.dll")]
+        public static extern bool SetWindowPlacement(IntPtr hWnd, in WINDOWPLACEMENT windowPlacement);
+        [DllImport("user32.dll")]
         public static extern bool SetFocus(IntPtr hWnd);
         [DllImport("user32.dll")]
         public static extern IntPtr GetFocus();
@@ -2240,6 +2285,14 @@ public struct TRACKMOUSEEVENT
             public int dwHoverTime;
         }
 
+        [Flags]
+        public enum WindowPlacementFlags : uint
+        {
+            SetMinPosition = 0x0001,
+            RestoreToMaximized = 0x0002,
+            AsyncWindowPlacement = 0x0004,
+        }
+
         [StructLayout(LayoutKind.Sequential)]
         public struct WINDOWPLACEMENT
         {
@@ -2254,7 +2307,7 @@ public struct WINDOWPLACEMENT
             /// <summary>
             /// Specifies flags that control the position of the minimized window and the method by which the window is restored.
             /// </summary>
-            public int Flags;
+            public WindowPlacementFlags Flags;
 
             /// <summary>
             /// The current show state of the window.

src/Windows/Avalonia.Win32/PlatformConstants.cs

@@ -8,6 +8,10 @@ internal static class PlatformConstants
         public const string CursorHandleType = "HCURSOR";
 
         public static readonly Version Windows10 = new Version(10, 0);
+        /// <summary>
+        /// Windows 10 Anniversary Update
+        /// </summary>
+        public static readonly Version Windows10_1607 = new Version(10, 0, 1607);
         public static readonly Version Windows8 = new Version(6, 2);
         public static readonly Version Windows8_1 = new Version(6, 3);
         public static readonly Version Windows7 = new Version(6, 1);

src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs

@@ -131,7 +131,7 @@ protected virtual unsafe IntPtr AppWndProc(IntPtr hWnd, uint msg, IntPtr wParam,
                     {
                         _dpi = (uint)wParam >> 16;
                         var newDisplayRect = Marshal.PtrToStructure<RECT>(lParam);
-                        _scaling = _dpi / 96.0;
+                        _scaling = _dpi / StandardDpi;
                         RefreshIcon();
                         ScalingChanged?.Invoke(_scaling);
 
@@ -613,14 +613,6 @@ protected virtual unsafe IntPtr AppWndProc(IntPtr hWnd, uint msg, IntPtr wParam,
                     {
                         var size = (SizeCommand)wParam;
 
-                        if (Resized != null &&
-                            (size == SizeCommand.Restored ||
-                             size == SizeCommand.Maximized))
-                        {
-                            var clientSize = new Size(ToInt32(lParam) & 0xffff, ToInt32(lParam) >> 16);
-                            Resized(clientSize / RenderScaling, _resizeReason);
-                        }
-
                         var windowState = size switch
                         {
                             SizeCommand.Maximized => WindowState.Maximized,
@@ -629,10 +621,20 @@ protected virtual unsafe IntPtr AppWndProc(IntPtr hWnd, uint msg, IntPtr wParam,
                             _ => WindowState.Normal,
                         };
 
-                        if (windowState != _lastWindowState)
+                        var stateChanged = windowState != _lastWindowState;
+                        _lastWindowState = windowState;
+
+                        if (Resized != null &&
+                            (size == SizeCommand.Restored ||
+                             size == SizeCommand.Maximized))
                         {
-                            _lastWindowState = windowState;
+                            var clientSize = new Size(ToInt32(lParam) & 0xffff, ToInt32(lParam) >> 16);
+                            Resized(clientSize / RenderScaling, _resizeReason);
+                        }
 
+
+                        if (stateChanged)
+                        {
                             var newWindowProperties = _windowProperties;
 
                             newWindowProperties.WindowState = windowState;

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -28,6 +28,7 @@
 using Avalonia.Threading;
 using static Avalonia.Controls.Platform.IWin32OptionsTopLevelImpl;
 using static Avalonia.Controls.Platform.Win32SpecificOptions;
+using Avalonia.Logging;
 
 namespace Avalonia.Win32
 {
@@ -54,6 +55,11 @@ internal partial class WindowImpl : IWindowImpl, EglGlPlatformSurface.IEglWindow
                 { WindowEdge.West, HitTestValues.HTLEFT }
             };
 
+        /// <summary>
+        /// The Windows DPI which equates to a <see cref="RenderScaling"/> of 1.0.
+        /// </summary>
+        public const double StandardDpi = 96;
+
         private SavedWindowInfo _savedWindowInfo;
         private bool _isFullScreenActive;
         private bool _isClientAreaExtended;
@@ -287,8 +293,7 @@ public WindowState WindowState
                     return WindowState.FullScreen;
                 }
 
-                var placement = default(WINDOWPLACEMENT);
-                GetWindowPlacement(_hwnd, ref placement);
+                GetWindowPlacement(_hwnd, out var placement);
 
                 return placement.ShowCmd switch
                 {
@@ -559,29 +564,59 @@ public void SetMinMaxSize(Size minSize, Size maxSize)
 
         public void Resize(Size value, WindowResizeReason reason)
         {
-            if (WindowState != WindowState.Normal)
-                return;
-
             int requestedClientWidth = (int)(value.Width * RenderScaling);
             int requestedClientHeight = (int)(value.Height * RenderScaling);
 
-            GetClientRect(_hwnd, out var clientRect);
+            GetClientRect(_hwnd, out var currentClientRect);
+            if (currentClientRect.Width == requestedClientWidth && currentClientRect.Height == requestedClientHeight)
+            {
+                // Don't update our window position if the client size is already correct. This leads to Windows updating our
+                // "normal position" (i.e. restored bounds) to match our maximised or areo snap size, which is incorrect behaviour.
+                // We only want to proceed with this method if the new size is coming from Avalonia.
+                return;
+            }
 
-            // do comparison after scaling to avoid rounding issues
-            if (requestedClientWidth != clientRect.Width || requestedClientHeight != clientRect.Height)
+            if (_lastWindowState == WindowState.FullScreen)
             {
-                GetWindowRect(_hwnd, out var windowRect);
+                // Fullscreen mode is really a restored window without a frame filling the whole monitor.
+                // It doesn't make sense to resize the window in this state, so ignore this request.
+                Logger.TryGet(LogEventLevel.Warning, LogArea.Win32Platform)?.Log(this, "Ignoring resize event on fullscreen window.");
+                return;
+            }
 
-                using var scope = SetResizeReason(reason);
-                SetWindowPos(
-                    _hwnd,
-                    IntPtr.Zero,
-                    0,
-                    0,
-                    requestedClientWidth + (_isClientAreaExtended ? 0 : windowRect.Width - clientRect.Width),
-                    requestedClientHeight + (_isClientAreaExtended ? 0 : windowRect.Height - clientRect.Height),
-                    SetWindowPosFlags.SWP_RESIZE);
+            GetWindowPlacement(_hwnd, out var windowPlacement);
+
+            var clientScreenOrigin = new POINT();
+            ClientToScreen(_hwnd, ref clientScreenOrigin);
+
+            var requestedClientRect = new RECT
+            {
+                left = clientScreenOrigin.X,
+                right = clientScreenOrigin.X + requestedClientWidth,
+
+                top = clientScreenOrigin.Y,
+                bottom = clientScreenOrigin.Y + requestedClientHeight,
+            };
+
+            var requestedWindowRect = _isClientAreaExtended ? requestedClientRect : ClientRectToWindowRect(requestedClientRect);
+
+            if (requestedWindowRect.Width == windowPlacement.NormalPosition.Width && requestedWindowRect.Height == windowPlacement.NormalPosition.Height)
+            {
+                return;
             }
+
+            windowPlacement.NormalPosition = requestedWindowRect;
+
+            windowPlacement.ShowCmd = _lastWindowState switch
+            {
+                WindowState.Minimized => ShowWindowCommand.ShowMinNoActive,
+                WindowState.Maximized => ShowWindowCommand.ShowMaximized,
+                WindowState.Normal => ShowWindowCommand.ShowNoActivate,
+                _ => throw new NotImplementedException(),
+            };
+
+            using var scope = SetResizeReason(reason);
+            SetWindowPlacement(_hwnd, in windowPlacement);
         }
 
         public void Activate()
@@ -913,7 +948,7 @@ private void CreateWindow()
                     out _dpi,
                     out _) == 0)
                 {
-                    _scaling = _dpi / 96.0;
+                    _scaling = _dpi / StandardDpi;
                 }
             }
         }
@@ -1473,6 +1508,23 @@ private static void EnableCloseButton(IntPtr hwnd)
                            MF_BYCOMMAND | MF_ENABLED);
         }
 
+        private RECT ClientRectToWindowRect(RECT clientRect, WindowStyles? styleOverride = null, WindowStyles? extendedStyleOverride = null)
+        {
+            var style = styleOverride ?? GetStyle();
+            var extendedStyle = extendedStyleOverride ?? GetExtendedStyle();
+
+            var result = Win32Platform.WindowsVersion < PlatformConstants.Windows10_1607
+                ? AdjustWindowRectEx(ref clientRect, (uint)style, false, (uint)extendedStyle)
+                : AdjustWindowRectExForDpi(ref clientRect, style, false, extendedStyle, (uint)(RenderScaling * StandardDpi));
+
+            if (!result)
+            {
+                Marshal.ThrowExceptionForHR(Marshal.GetHRForLastWin32Error());
+            }
+
+            return clientRect;
+        }
+
 #if USE_MANAGED_DRAG
         private Point ScreenToClient(Point point)
         {