re-create NSWindow if we call show and need to swap out for a dialog.

native/Avalonia.Native/src/OSX/AvnWindow.mm

@@ -433,5 +433,9 @@
     }
 }
 
+- (void)disconnectParent {
+    _parent = nullptr;
+}
+
 @end
 

native/Avalonia.Native/src/OSX/WindowBaseImpl.h

@@ -123,6 +123,7 @@ protected:
 
 private:
     void CreateNSWindow (bool isDialog);
+    void CleanNSWindow ();
     void InitialiseNSWindow ();
 };
 

native/Avalonia.Native/src/OSX/WindowBaseImpl.mm

@@ -525,14 +525,25 @@ void WindowBaseImpl::UpdateStyle() {
     [Window setStyleMask:GetStyle()];
 }
 
+void WindowBaseImpl::CleanNSWindow() {
+    if(Window != nullptr) {
+        [GetWindowProtocol() disconnectParent];
+        [Window close];
+        Window = nullptr;
+    }
+}
+
 void WindowBaseImpl::CreateNSWindow(bool isDialog) {
-    if(Window == nullptr) {
-        if(isDialog)
-        {
+    if (isDialog) {
+        if (![Window isKindOfClass:[AvnPanel class]]) {
+            CleanNSWindow();
+
             Window = [[AvnPanel alloc] initWithParent:this contentRect:NSRect{0, 0, lastSize} styleMask:GetStyle()];
         }
-        else
-        {
+    } else {
+        if (![Window isKindOfClass:[AvnWindow class]]) {
+            CleanNSWindow();
+
             Window = [[AvnWindow alloc] initWithParent:this contentRect:NSRect{0, 0, lastSize} styleMask:GetStyle()];
         }
     }

native/Avalonia.Native/src/OSX/WindowProtocol.h

@@ -20,5 +20,6 @@
 
 -(double) getExtendedTitleBarHeight;
 -(void) setIsExtended:(bool)value;
+-(void) disconnectParent;
 -(bool) isDialog;
 @end