fix bug - popup windows show hiding logic

CursorLang.Agent/Windows/NativeWindow.cs

@@ -67,9 +67,14 @@ internal abstract class NativeWindow : IDisposable
 
     /// <summary>
     /// A message for this window. Returning <c>false</c> passes it to
-    /// <c>DefWindowProcW</c>, which is what the vast majority of messages want.
+    /// <c>DefWindowProcW</c>, which is what the vast majority of messages want — and
+    /// what all of them want for a window whose content is set from the outside.
     /// </summary>
-    protected abstract bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result);
+    protected virtual bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result)
+    {
+        result = IntPtr.Zero;
+        return false;
+    }
 
     private static IntPtr StaticWindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam)
     {