Fix resize when maximised

Warn and ignore when a fullscreen window is resized

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
         }
 

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -30,6 +30,7 @@
 using Avalonia.Threading;
 using static Avalonia.Controls.Platform.IWin32OptionsTopLevelImpl;
 using static Avalonia.Controls.Platform.Win32SpecificOptions;
+using Avalonia.Logging;
 
 namespace Avalonia.Win32
 {
@@ -572,6 +573,14 @@ public void Resize(Size value, WindowResizeReason reason)
                 return;
             }
 
+            if (_lastWindowState == WindowState.FullScreen)
+            {
+                // 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;
+            }
+
             GetWindowPlacement(_hwnd, out var windowPlacement);
 
             var clientScreenOrigin = new POINT();
@@ -598,7 +607,9 @@ public void Resize(Size value, WindowResizeReason reason)
             windowPlacement.ShowCmd = _lastWindowState switch
             {
                 WindowState.Minimized => ShowWindowCommand.ShowMinNoActive,
-                _ => ShowWindowCommand.ShowNoActivate,
+                WindowState.Maximized => ShowWindowCommand.ShowMaximized,
+                WindowState.Normal => ShowWindowCommand.ShowNoActivate,
+                _ => throw new NotImplementedException(),
             };
 
             using var scope = SetResizeReason(reason);