Make sure focus lost is delayed until IMM32 has finished up the composition on kill focus (#15907)

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

@@ -19,6 +19,8 @@ namespace Avalonia.Win32
 {
     internal partial class WindowImpl
     {
+        private bool _killFocusRequested;
+
         [SuppressMessage("Microsoft.StyleCop.CSharp.NamingRules", "SA1305:FieldNamesMustNotUseHungarianNotation",
             Justification = "Using Win32 naming for consistency.")]
         [UnconditionalSuppressMessage("Trimming", "IL2026", Justification = "We do .NET COM interop availability checks")]
@@ -718,7 +720,15 @@ namespace Avalonia.Win32
                     }
 
                 case WindowsMessage.WM_KILLFOCUS:
-                    LostFocus?.Invoke();
+                    if (Imm32InputMethod.Current.IsComposing)
+                    {
+                        _killFocusRequested = true;
+                    }
+                    else
+                    {
+                        LostFocus?.Invoke();
+                    }
+                   
                     break;
 
                 case WindowsMessage.WM_INPUTLANGCHANGE:
@@ -763,6 +773,13 @@ namespace Avalonia.Win32
                     {
                         Imm32InputMethod.Current.HandleCompositionEnd(timestamp);
 
+                        if (_killFocusRequested)
+                        {
+                            LostFocus?.Invoke();
+
+                            _killFocusRequested = false;
+                        }
+
                         return IntPtr.Zero;
                     }
                 case WindowsMessage.WM_GETOBJECT: