Fix ShowInTaskbar on Win32.

To hide the taskbar icon on win32 we need to parent the window to a hidden window.

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -85,6 +85,7 @@ namespace Avalonia.Win32
         private ExtendClientAreaChromeHints _extendChromeHints = ExtendClientAreaChromeHints.Default;
         private bool _isCloseRequested;
         private bool _shown;
+        private WindowImpl _hiddenWindow;
 
         public WindowImpl()
         {
@@ -581,7 +582,7 @@ namespace Avalonia.Win32
         public void SetParent(IWindowImpl parent)
         {
             _parent = (WindowImpl)parent;
-            SetWindowLongPtr(_hwnd, (int)WindowLongParam.GWL_HWNDPARENT, _parent._hwnd);
+            SetWindowLongPtr(_hwnd, (int)WindowLongParam.GWL_HWNDPARENT, _parent?._hwnd ?? IntPtr.Zero);
         }
 
         public void SetEnabled(bool enable) => EnableWindow(_hwnd, enable);
@@ -755,6 +756,11 @@ namespace Avalonia.Win32
             }
         }
 
+        private WindowImpl GetOrCreateHiddenWindow()
+        {
+            return _hiddenWindow ??= new WindowImpl();
+        }
+
         private void CreateDropTarget()
         {
             var odt = new OleDropTarget(this, _owner);
@@ -1094,16 +1100,35 @@ namespace Avalonia.Win32
                 if (newProperties.ShowInTaskbar)
                 {
                     exStyle |= WindowStyles.WS_EX_APPWINDOW;
+
+                    if (_hiddenWindow is object && _parent == _hiddenWindow)
+                    {
+                        // Can't enable the taskbar icon by clearing the parent window unless the window
+                        // is hidden. Hide the window and show it again with the same activation state
+                        // when we've finished. Interestingly it seems to work fine the other way.
+                        var shown = IsWindowVisible(_hwnd);
+                        var activated = GetActiveWindow() == _hwnd;
+
+                        if (shown)
+                            Hide();
+
+                        SetParent(null);
+
+                        if (shown)
+                            Show(activated);
+                    }
+
                 }
                 else
                 {
+                    // To hide a non-owned window's taskbar icon we need to parent it to a hidden window.
+                    if (_parent is null)
+                        SetParent(GetOrCreateHiddenWindow());
+
                     exStyle &= ~WindowStyles.WS_EX_APPWINDOW;
                 }
 
                 SetExtendedStyle(exStyle);
-
-                // TODO: To hide non-owned window from taskbar we need to parent it to a hidden window.
-                // Otherwise it will still show in the taskbar.
             }
 
             WindowStyles style;