diff --git a/OpenNest.WinForms.Tests/Forms/MainFormCharacterizationTests.cs b/OpenNest.WinForms.Tests/Forms/MainFormCharacterizationTests.cs
new file mode 100644
index 00000000..919958b5
--- /dev/null
+++ b/OpenNest.WinForms.Tests/Forms/MainFormCharacterizationTests.cs
@@ -0,0 +1,130 @@
+using System.Reflection;
+using OpenNest.Controls;
+using OpenNest.Forms;
+using OpenNest.Geometry;
+
+namespace OpenNest.WinForms.Tests.Forms;
+
+// Pins MainForm behaviour that its refactor must preserve: nest naming and the deliberately
+// different busy guards. Reads Application.OpenForms, so it must not overlap fill-lifetime tests.
+[Collection("Fill operation lifetime")]
+public class MainFormCharacterizationTests
+{
+    [Theory]
+    [InlineData(0, "N26-222")]
+    [InlineData(1, "N26-223")]
+    [InlineData(29, "N26-22Z")]
+    [InlineData(30, "N26-232")]
+    [InlineData(900, "N26-322")]
+    [InlineData(27000, "N26-3222")]
+    public void NestNamesUseTwoDigitYearAndPaddedBase30Sequence(int id, string expected) =>
+        StaTestThread.Run(() =>
+        {
+            using var host = new Host();
+            var name = (string)typeof(MainForm)
+                .GetMethod("GetNestName", BindingFlags.Instance | BindingFlags.NonPublic)!
+                .Invoke(host, [new DateTime(2026, 10, 17), id])!;
+
+            Assert.Equal(expected, name);
+        }, TimeSpan.FromMinutes(1), "The STA test did not complete.");
+
+    [Fact]
+    public void BusyGuardsDifferInWhichOperationsTheyCount() => StaTestThread.Run(() =>
+    {
+        using var host = new Host();
+        host.Show();
+        var nest = new Nest("guards");
+        var plate = nest.CreatePlate();
+        plate.Size = new Size(100, 100);
+        plate.Parts.Add(new Part(new Drawing("left", Square()), new Vector(1, 1)));
+        plate.Parts.Add(new Part(new Drawing("right", Square()), new Vector(12, 1)));
+        using var editor = new EditNestForm(nest) { MdiParent = host };
+        editor.PlateView.SetOverlapAutoCheck(null);
+        editor.Show();
+        editor.Activate();
+        editor.PlateManager.LoadAt(0);
+        var other = new Nest("other");
+        other.CreatePlate().Size = new Size(100, 100);
+        using var otherEditor = new EditNestForm(other) { MdiParent = host };
+        otherEditor.PlateView.SetOverlapAutoCheck(null);
+        otherEditor.Show();
+        otherEditor.PlateManager.LoadAt(0);
+        var views = new[] { editor.PlateView };
+
+        Assert.Equal(new Guards(false, false, false, false), Read(host, editor, views));
+
+        // Whole-job nesting blocks every guard.
+        SetField(host, "nestingInProgress", true);
+        Assert.Equal(new Guards(true, true, true, true), Read(host, editor, views));
+        SetField(host, "nestingInProgress", false);
+
+        // A database save only blocks the report snapshot.
+        SetField(host, "databaseSaveInProgress", true);
+        Assert.Equal(new Guards(false, false, false, true), Read(host, editor, views));
+        SetField(host, "databaseSaveInProgress", false);
+
+        // A fill on a view of the nest blocks the plate guards but not the report.
+        SetBackingField(editor.PlateView, "IsFillInProgress", true);
+        try
+        {
+            Assert.Equal(new Guards(true, true, true, false), Read(host, editor, views));
+        }
+        finally
+        {
+            SetBackingField(editor.PlateView, "IsFillInProgress", false);
+        }
+
+        // A fill on another nest's view blocks nothing for this nest.
+        SetBackingField(otherEditor.PlateView, "IsFillInProgress", true);
+        try
+        {
+            Assert.Equal(new Guards(false, false, false, false), Read(host, editor, views));
+        }
+        finally
+        {
+            SetBackingField(otherEditor.PlateView, "IsFillInProgress", false);
+        }
+
+        // A busy plate action (selection) blocks only the cutting-plan guard.
+        editor.PlateView.SelectAll();
+        Assert.True(editor.PlateView.Actions.CurrentAction?.IsBusy() == true);
+        Assert.Equal(new Guards(false, true, false, false), Read(host, editor, views));
+    }, TimeSpan.FromMinutes(1), "The STA test did not complete.");
+
+    private sealed record Guards(bool ExpandSpacing, bool CuttingPlan, bool Clamps, bool NestJob);
+
+    private static Guards Read(MainForm host, EditNestForm editor, PlateView[] views) => new(
+        host.IsExpandSpacingBusy(editor),
+        host.IsCuttingPlanBusy(views),
+        (bool)Invoke(host, "ClampsInUse", editor.Nest),
+        (bool)Invoke(host, "IsNestJobBusy"));
+
+    private static object Invoke(MainForm host, string name, params object[] args) =>
+        typeof(MainForm).GetMethod(name, BindingFlags.Instance | BindingFlags.NonPublic)!.Invoke(host, args)!;
+
+    private static void SetField(MainForm host, string name, object value) =>
+        typeof(MainForm).GetField(name, BindingFlags.Instance | BindingFlags.NonPublic)!.SetValue(host, value);
+
+    private static void SetBackingField(object target, string property, object value) =>
+        target.GetType()
+            .GetField($"<{property}>k__BackingField", BindingFlags.Instance | BindingFlags.NonPublic)!
+            .SetValue(target, value);
+
+    private static CNC.Program Square()
+    {
+        var program = new CNC.Program();
+        program.MoveTo(0, 0);
+        program.LineTo(0, 10);
+        program.LineTo(10, 10);
+        program.LineTo(10, 0);
+        program.LineTo(0, 0);
+        return program;
+    }
+
+    private sealed class Host : MainForm
+    {
+        // No startup migrations/automatic new document, and no settings save at close.
+        protected override void OnLoad(EventArgs e) { }
+        protected override void OnClosing(System.ComponentModel.CancelEventArgs e) { }
+    }
+}