Fixes Issue 4980: Message loop exit when GetMessage < 0

src/Avalonia.Base/Logging/LogArea.cs

@@ -34,5 +34,10 @@ namespace Avalonia.Logging
         /// The log event comes from the control system.
         /// </summary>
         public const string Control = "Control";
+
+        /// <summary>
+        /// The log evevnt come from Win32Platform
+        /// </summary>
+        public const string Win32Platform = nameof(Win32Platform);
     }
 }

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

@@ -931,8 +931,8 @@ namespace Avalonia.Win32.Interop
         [DllImport("user32.dll", EntryPoint = "MapVirtualKeyW")]
         public static extern uint MapVirtualKey(uint uCode, uint uMapType);
 
-        [DllImport("user32.dll", EntryPoint = "GetMessageW")]
-        public static extern sbyte GetMessage(out MSG lpMsg, IntPtr hWnd, uint wMsgFilterMin, uint wMsgFilterMax);
+        [DllImport("user32.dll", EntryPoint = "GetMessageW",SetLastError = true)]
+        public static extern int GetMessage(out MSG lpMsg, IntPtr hWnd, uint wMsgFilterMin, uint wMsgFilterMax);
 
         [DllImport("user32.dll")]
         public static extern int GetMessageTime();

src/Windows/Avalonia.Win32/Win32Platform.cs

@@ -137,13 +137,18 @@ namespace Avalonia.Win32
 
         public void RunLoop(CancellationToken cancellationToken)
         {
-            while (!cancellationToken.IsCancellationRequested)
+            var result = 0;
+            while (!cancellationToken.IsCancellationRequested 
+                && (result = UnmanagedMethods.GetMessage(out var msg, IntPtr.Zero, 0, 0)) > 0)
             {
-                UnmanagedMethods.MSG msg;
-                UnmanagedMethods.GetMessage(out msg, IntPtr.Zero, 0, 0);
                 UnmanagedMethods.TranslateMessage(ref msg);
                 UnmanagedMethods.DispatchMessage(ref msg);
             }
+            if (result < 0)
+            {
+                Logging.Logger.TryGet(Logging.LogEventLevel.Error, Logging.LogArea.Win32Platform)
+                    ?.Log(this, "Unmanaged error in message loop. Error Code: {0}", Marshal.GetLastWin32Error());
+            }
         }
 
         public IDisposable StartTimer(DispatcherPriority priority, TimeSpan interval, Action callback)