ensure owner topmost flag is set if its topmost when showing a owned window

Author: emmauss

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -708,6 +708,8 @@ public void SetParent(IWindowImpl? parent)
             _hiddenWindowIsParent = parentHwnd == OffscreenParentWindow.Handle;
 
             SetWindowLongPtr(_hwnd, (int)WindowLongParam.GWL_HWNDPARENT, parentHwnd);
+
+            (parent as WindowImpl)?.EnsureTopmost();
         }
 
         public void SetEnabled(bool enable) => EnableWindow(_hwnd, enable);
@@ -860,6 +862,17 @@ public void SetTopmost(bool value)
             _topmost = value;
         }
 
+        private void EnsureTopmost()
+        {
+            if(_topmost)
+            {
+                SetWindowPos(_hwnd,
+                    WindowPosZOrder.HWND_TOPMOST,
+                    0, 0, 0, 0,
+                    SetWindowPosFlags.SWP_NOMOVE | SetWindowPosFlags.SWP_NOSIZE | SetWindowPosFlags.SWP_NOACTIVATE);
+            }
+        }
+
         public unsafe void SetFrameThemeVariant(PlatformThemeVariant themeVariant)
         {
             _currentThemeVariant = themeVariant;