diff --git a/IoListTestingWindow.P1FirstPaint.cs b/IoListTestingWindow.P1FirstPaint.cs
new file mode 100644
index 000000000..7cfa58263
--- /dev/null
+++ b/IoListTestingWindow.P1FirstPaint.cs
@@ -0,0 +1,40 @@
+using System.Windows;
+
+namespace ArIED61850Tester;
+
+/// <summary>
+/// P1 first-paint guard for the FAT workspace.
+///
+/// The golden/P0 workspace still declares legacy relay-time columns in XAML and replaces
+/// them with the V2 schema from the ContentRendered path. Installing the existing V2
+/// workspace during Loaded keeps the visual tree available while ensuring the first
+/// visible FAT paint already uses LIVE VALUE / VALUE 1 / VALUE 2.
+///
+/// InstallFatV2WorkspaceUx is intentionally reused and remains idempotent. No runtime
+/// virtualization, protocol acquisition, command, evidence, or ARIEC61850 behavior is
+/// changed here.
+/// </summary>
+public partial class IoListTestingWindow
+{
+    private static readonly bool P1FirstPaintRegistered = RegisterP1FirstPaint();
+
+    private static bool RegisterP1FirstPaint()
+    {
+        EventManager.RegisterClassHandler(
+            typeof(IoListTestingWindow),
+            FrameworkElement.LoadedEvent,
+            new RoutedEventHandler(P1FirstPaint_Loaded));
+        return true;
+    }
+
+    private static void P1FirstPaint_Loaded(object sender, RoutedEventArgs e)
+    {
+        if (sender is not IoListTestingWindow window ||
+            !ReferenceEquals(e.OriginalSource, window))
+        {
+            return;
+        }
+
+        window.InstallFatV2WorkspaceUx();
+    }
+}
diff --git a/tests/ARSAS.Tests/IoFatP1FirstPaintRegressionTests.cs b/tests/ARSAS.Tests/IoFatP1FirstPaintRegressionTests.cs
new file mode 100644
index 000000000..cb65b82df
--- /dev/null
+++ b/tests/ARSAS.Tests/IoFatP1FirstPaintRegressionTests.cs
@@ -0,0 +1,61 @@
+namespace ARSAS.Tests;
+
+public sealed class IoFatP1FirstPaintRegressionTests
+{
+    [Fact]
+    public void P1_InstallsExistingFatV2SchemaOnLoadedBeforeFirstVisibleRender()
+    {
+        var source = File.ReadAllText(FindRepoFile("IoListTestingWindow.P1FirstPaint.cs"));
+        var handlerStart = source.IndexOf("private static void P1FirstPaint_Loaded", StringComparison.Ordinal);
+        Assert.True(handlerStart >= 0);
+        var handler = source[handlerStart..];
+
+        Assert.Contains("FrameworkElement.LoadedEvent", source, StringComparison.Ordinal);
+        Assert.Contains("window.InstallFatV2WorkspaceUx();", handler, StringComparison.Ordinal);
+        Assert.Contains("ReferenceEquals(e.OriginalSource, window)", handler, StringComparison.Ordinal);
+        Assert.DoesNotContain("ContentRendered", handler, StringComparison.Ordinal);
+        Assert.DoesNotContain("Dispatcher.BeginInvoke", handler, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void P1_V2InstallerIsIdempotentAndDefinesOnlyApprovedEvidenceColumns()
+    {
+        var source = File.ReadAllText(FindRepoFile("IoListTestingWindow.FatV2Ux.cs"));
+
+        Assert.Contains("if (!_fatV2UxInstalled)", source, StringComparison.Ordinal);
+        Assert.Contains("_fatV2UxInstalled = true;", source, StringComparison.Ordinal);
+        Assert.Contains("grid.Columns.Clear();", source, StringComparison.Ordinal);
+        Assert.Contains("Header = \"LIVE VALUE\"", source, StringComparison.Ordinal);
+        Assert.Contains("FatValueSlot.Value1", source, StringComparison.Ordinal);
+        Assert.Contains("FatValueSlot.Value2", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("\"ON RELAY TIME\"", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("\"OFF RELAY TIME\"", source, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void P1_FirstPaintGuardDoesNotTouchVirtualizationProtocolOrEvidence()
+    {
+        var source = File.ReadAllText(FindRepoFile("IoListTestingWindow.P1FirstPaint.cs"));
+
+        Assert.DoesNotContain("SetVirtualizationMode", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("SetIsVirtualizing", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("ReadObject", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("ReadAsync", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("CaptureCurrentEvidence", source, StringComparison.Ordinal);
+    }
+
+    private static string FindRepoFile(string relativePath)
+    {
+        DirectoryInfo? directory = new(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            var candidate = Path.Combine(directory.FullName, relativePath);
+            if (File.Exists(candidate))
+                return candidate;
+            directory = directory.Parent;
+        }
+
+        throw new FileNotFoundException(
+            $"Could not locate repository file '{relativePath}' from '{AppContext.BaseDirectory}'.");
+    }
+}