Fixes #issue. Dialog renders with Base scheme instead of Dialog scheme when running

Terminal.Gui/Views/DialogTResult.cs

@@ -354,6 +354,14 @@ private void SetStyle ()
     {
         if (IsRunning)
         {
+            // When running, restore to Dialog scheme only if it was set to Base by SetStyle
+            // (i.e., the scheme was not explicitly overridden before running, e.g. by
+            // MessageBox.ErrorQuery which sets SchemeName = "Error" before calling app.Run).
+            if (SchemeName == SchemeManager.SchemesToSchemeName (Schemes.Base))
+            {
+                SchemeName = SchemeManager.SchemesToSchemeName (Schemes.Dialog);
+            }
+
             Arrangement |= ViewArrangement.Movable | ViewArrangement.Resizable | ViewArrangement.Overlapped;
         }
         else

Tests/UnitTestsParallelizable/Views/DialogTests.cs

@@ -412,6 +412,45 @@ public void ShadowStyle_Can_Be_Changed ()
         dialog.Dispose ();
     }
 
+    // Copilot
+    [Fact]
+    public void SchemeName_IsBase_WhenNotRunning ()
+    {
+        // When a Dialog is not running, it should use the Base scheme (not Dialog)
+        Dialog dialog = new ();
+
+        Assert.Equal (SchemeManager.SchemesToSchemeName (Schemes.Base), dialog.SchemeName);
+
+        dialog.Dispose ();
+    }
+
+    // Copilot
+    [Fact]
+    public void SchemeName_IsDialog_WhenRunning ()
+    {
+        using IApplication app = Application.Create ();
+        app.Init (DriverRegistry.Names.ANSI);
+
+        using Dialog dialog = new ();
+
+        string? schemeNameWhileRunning = null;
+
+        app.Iteration += AppOnIteration;
+        app.Run (dialog);
+        app.Iteration -= AppOnIteration;
+
+        Assert.Equal (SchemeManager.SchemesToSchemeName (Schemes.Dialog), schemeNameWhileRunning);
+
+        return;
+
+        void AppOnIteration (object? sender, EventArgs<IApplication?> e)
+        {
+            schemeNameWhileRunning = dialog.SchemeName;
+            app.Iteration -= AppOnIteration;
+            app.RequestStop ();
+        }
+    }
+
     [Fact]
     public void Text_Property ()
     {