P0-D: live signal viewport layout guard and field-evidence UX hardening

MainWindow.xaml

@@ -607,12 +607,16 @@
                                         </Border>
                                     </Grid>
 
-                                    <DataGrid Grid.Row="2" ItemsSource="{Binding SelectedDevice.Points}" IsReadOnly="True"
+                                    <DataGrid x:Name="ExplorerLiveValuesGrid" Grid.Row="2"
+                                              ItemsSource="{Binding SelectedDevice.Points}" IsReadOnly="True"
                                               Loaded="ExplorerLiveGrid_Loaded"
                                               Style="{StaticResource ModernDataGrid}" FrozenColumnCount="2"
+                                              MinHeight="0" VerticalAlignment="Stretch"
                                               EnableRowVirtualization="True" EnableColumnVirtualization="True"
                                               VirtualizingPanel.IsVirtualizing="True" VirtualizingPanel.VirtualizationMode="Recycling"
-                                              ScrollViewer.CanContentScroll="True" ScrollViewer.HorizontalScrollBarVisibility="Auto">
+                                              ScrollViewer.CanContentScroll="True"
+                                              ScrollViewer.VerticalScrollBarVisibility="Auto"
+                                              ScrollViewer.HorizontalScrollBarVisibility="Auto">
                                         <DataGrid.RowStyle>
                                             <Style TargetType="DataGridRow" BasedOn="{StaticResource {x:Type DataGridRow}}">
                                                 <Style.Triggers>

Services/NativeIec61850Client.cs

@@ -4360,6 +4360,25 @@ private static string NormalizeFunctionalConstraint(string functionalConstraint,
 
     private Iec61850ReadValue? ProjectReadValue(ArMms.MmsDataValue? value, string dataType, string readReference, string requestedReference)
     {
+        // A successful, *typed* MMS OCTET STRING of length zero is a
+        // legitimate observation, not a missing Read or an "unavailable"
+        // scalar. This matters for optional origin.orIdent attributes.
+        // Do not apply to VisibleString, Unknown BER or malformed responses.
+        if (value?.Kind == ArMms.MmsDataKind.OctetString &&
+            value.RawValue.Count == 0 &&
+            (dataType.StartsWith("Octet", StringComparison.OrdinalIgnoreCase) ||
+             dataType.Equals("OctetString", StringComparison.OrdinalIgnoreCase)))
+        {
+            return new Iec61850ReadValue
+            {
+                Value = "Empty OCTET STRING (0 bytes)",
+                DisplayValue = "Empty OCTET STRING (0 bytes)",
+                SourceReference = requestedReference,
+                ReadReference = readReference,
+                Projection = "typed-empty-octet-string"
+            };
+        }
+
         // Explicit object-level DataSet members must be projected before generic
         // first-scalar semantic binding. Otherwise a three-phase THD object collapses
         // to phase A and loses the B/C values carried by the same MMS structure.

tests/ARSAS.Tests/P0DLiveSignalViewportContractTests.cs

@@ -0,0 +1,51 @@
+using System.Xml.Linq;
+
+namespace ARSAS.Tests;
+
+/// <summary>
+/// Compile-time WPF layout contract; runtime DPI screenshot acceptance remains
+/// a separate Windows integration test, never asserted by XML inspection.
+/// </summary>
+public sealed class P0DLiveSignalViewportContractTests
+{
+    [Fact]
+    public void LiveSignalViewport_RemainsInStarRegion_WithIndependentScrolling()
+    {
+        var root = FindRepositoryRoot();
+        var xaml = XDocument.Load(Path.Combine(root, "MainWindow.xaml"));
+        var grid = xaml.Descendants().Single(x => x.Name.LocalName == "DataGrid" &&
+            (string?)x.Attribute(XName.Get("Name",
+                "http://schemas.microsoft.com/winfx/2006/xaml")) == "ExplorerLiveValuesGrid");
+        Assert.Equal("2", (string?)grid.Attribute("Grid.Row"));
+        Assert.Equal("Stretch", (string?)grid.Attribute("VerticalAlignment"));
+        Assert.Equal("0", (string?)grid.Attribute("MinHeight"));
+        Assert.Equal("Auto", (string?)grid.Attribute(XName.Get(
+            "ScrollViewer.VerticalScrollBarVisibility",
+            "http://schemas.microsoft.com/winfx/2006/xaml/presentation")));
+        Assert.Equal("True", (string?)grid.Attribute(XName.Get(
+            "VirtualizingPanel.IsVirtualizing",
+            "http://schemas.microsoft.com/winfx/2006/xaml/presentation")));
+        Assert.Equal("Recycling", (string?)grid.Attribute(XName.Get(
+            "VirtualizingPanel.VirtualizationMode",
+            "http://schemas.microsoft.com/winfx/2006/xaml/presentation")));
+        var container = grid.Parent;
+        Assert.NotNull(container);
+        var definitions = container!.Elements().First(x =>
+            x.Name.LocalName == "Grid.RowDefinitions")
+            .Elements().ToArray();
+        Assert.Equal("*", (string?)definitions[2].Attribute("Height"));
+        var outerGrid = container.Parent?.Parent;
+        Assert.NotNull(outerGrid);
+        Assert.Equal("Grid", outerGrid!.Name.LocalName);
+    }
+
+    private static string FindRepositoryRoot()
+    {
+        var folder = new DirectoryInfo(AppContext.BaseDirectory);
+        while (folder is not null &&
+               !File.Exists(Path.Combine(folder.FullName, "MainWindow.xaml")))
+            folder = folder.Parent;
+        return folder?.FullName ?? throw new InvalidOperationException(
+            "Test requires the tracked MainWindow.xaml.");
+    }
+}

tests/ARSAS.Tests/P0DTypedEmptyOctetEvidenceTests.cs

@@ -0,0 +1,49 @@
+using AR.Iec61850.Mms;
+using ArIED61850Tester.Models;
+using ArIED61850Tester.Services;
+using System.Reflection;
+
+namespace ARSAS.Tests;
+
+public sealed class P0DTypedEmptyOctetEvidenceTests
+{
+    [Fact]
+    public void ExactTypedEmptyOriginIdentifier_IsObserved_NotReadFailed()
+    {
+        var client = new NativeIec61850Client();
+        var method = typeof(NativeIec61850Client).GetMethod(
+            "ProjectReadValue", BindingFlags.NonPublic | BindingFlags.Instance);
+        Assert.NotNull(method);
+        const string reference = "LABPROT/LLN0.Mod.origin.orIdent";
+        var value = MmsDataValue.OctetString(ReadOnlySpan<byte>.Empty);
+        var read = method!.Invoke(client, [value, "Octet64", reference, reference])
+            as Iec61850ReadValue;
+        Assert.NotNull(read);
+        Assert.Equal("typed-empty-octet-string", read.Projection);
+        Assert.Equal("Empty OCTET STRING (0 bytes)", read.ToString());
+        Assert.True(IedBrowserReadValuePolicy.TryAcceptDisplay(
+            read.ToString(), out _, out _));
+        var ledger = new IedBrowserLiveValues();
+        var association = ledger.BeginAssociation();
+        Assert.True(ledger.PublishForAssociation(association, reference, "ST",
+            read.ToString(), "", "", "MMS Read"));
+        Assert.True(ledger.TryGet(reference, "ST", out var observed));
+        Assert.Equal("Empty OCTET STRING (0 bytes)", observed.Value);
+    }
+
+    [Fact]
+    public void EmptyTextOrUnknownTag_NeverGetsOctetWireAuthority()
+    {
+        using var client = new NativeIec61850Client();
+        var method = typeof(NativeIec61850Client).GetMethod(
+            "ProjectReadValue", BindingFlags.NonPublic | BindingFlags.Instance);
+        Assert.NotNull(method);
+        const string reference = "LABPROT/LLN0.Mod.origin.orIdent";
+        var emptyVisible = method!.Invoke(client,
+            [MmsDataValue.VisibleString(""), "Octet64", reference, reference])
+            as Iec61850ReadValue;
+        Assert.NotEqual("typed-empty-octet-string", emptyVisible?.Projection);
+        Assert.False(IedBrowserReadValuePolicy.TryAcceptDisplay(
+            "", out _, out _));
+    }
+}