simply applywm styles.

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -894,16 +894,20 @@ namespace Avalonia.Win32
 
             style = style | controlledFlags ^ controlledFlags;
 
-            style |= WindowStyles.WS_OVERLAPPEDWINDOW;
+            style |= WindowStyles.WS_OVERLAPPED |
+                       WindowStyles.WS_MINIMIZEBOX |
+                       WindowStyles.WS_MAXIMIZEBOX;
 
-            if (!_decorated)
+            if (_decorated)
             {
-                style ^= (WindowStyles.WS_CAPTION | WindowStyles.WS_SYSMENU);
+                style |= 
+                    WindowStyles.WS_CAPTION |
+                    WindowStyles.WS_SYSMENU;
             }
 
-            if (!_resizable)
+            if (_resizable)
             {
-                style ^= (WindowStyles.WS_SIZEFRAME);
+                style |= WindowStyles.WS_SIZEFRAME;
             }
 
             var oldThickness = BorderThickness;
@@ -936,7 +940,6 @@ namespace Avalonia.Win32
             UnmanagedMethods.SetWindowPos(_hwnd, IntPtr.Zero, (int)newRect.X, (int)newRect.Y, (int)newRect.Width,
                 (int)newRect.Height,
                 UnmanagedMethods.SetWindowPosFlags.SWP_NOZORDER | UnmanagedMethods.SetWindowPosFlags.SWP_NOACTIVATE | SetWindowPosFlags.SWP_FRAMECHANGED);
-
         }
 
         public void CanResize(bool value)