test(desktop): fail WinForms tests on UI-callback exceptions per STA thread

TestAssemblySetup set the unhandled-exception mode and a ThreadException
handler from a module initializer. WinForms stores both per thread
(ThreadContext/NativeWindow thread statics), so they applied only to the
runner thread that loaded the assembly, never to the STA threads the
tests run on. There, an exception escaping a window message or a
BeginInvoke/SynchronizationContext callback still opened a modal
ThreadExceptionDialog in an interactive session, or was silently ignored
otherwise; the handler would also have logged and swallowed it.

Replace the fifteen copied RunSta helpers with StaTestThread.Run, which
selects UnhandledExceptionMode.ThrowException on each STA thread before
it creates a window, so the exception propagates to the test body and
fails the test. Each class keeps its own timeout and message; all
threads are now background threads. Remove TestAssemblySetup.

StaTestThreadTests covers a throwing BeginInvoke callback through
StaTestThread (fails with the original exception) and, as a control,
the default mode on a plain STA thread (the exception goes to the
thread's handler and never reaches the test body). Cross-compiled on
Linux; Windows execution is the windows-desktop CI job.

OpenNest.WinForms.Tests/ActionSelectAreaCutOffTests.cs

@@ -1,5 +1,4 @@
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.Actions;
 using OpenNest.Controls;
@@ -86,19 +85,6 @@ public class ActionSelectAreaCutOffTests
         public void ToggleSelectionDirection() => OnKeyUp(new KeyEventArgs(Keys.Space));
     }
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { failure = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(60)), "The selection/fill test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(60), "The selection/fill test did not complete.");
 }

OpenNest.WinForms.Tests/ArchUnitsTests.cs

@@ -1,7 +1,6 @@
 using System.Drawing;
 using System.Globalization;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Text.RegularExpressions;
 using System.Windows.Forms;
 
@@ -48,19 +47,6 @@ public class ArchUnitsTests
 
     // Actual timeout execution is covered by the shared Regex mechanism in FractionTests;
     // these desktop tests deliberately avoid timing-sensitive oversized-input fixtures.
-    internal static void RunSta(Action action)
-    {
-        Exception? error = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { error = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(15)), "The STA test did not complete.");
-        if (error != null)
-            ExceptionDispatchInfo.Capture(error).Throw();
-    }
+    internal static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(15), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Controls/ProgramEditorControlTests.cs

@@ -1,6 +1,5 @@
 using System.Drawing;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Text.RegularExpressions;
 using System.Windows.Forms;
 using OpenNest.CNC;
@@ -292,19 +291,6 @@ public class ProgramEditorControlTests
     private static void ApplyHighlighting(ProgramEditorControl control) => typeof(ProgramEditorControl)
         .GetMethod("ApplyHighlighting", BindingFlags.Instance | BindingFlags.NonPublic)!.Invoke(control, null);
 
-    private static void RunSta(Action action)
-    {
-        Exception? error = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { error = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(30)), "The STA test did not complete.");
-        if (error != null)
-            ExceptionDispatchInfo.Capture(error).Throw();
-    }
+    private static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(30), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Controls/SeparatorPenLifetimeTests.cs

@@ -1,6 +1,5 @@
 using System.Drawing;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.Controls;
 
@@ -109,19 +108,6 @@ public class SeparatorPenLifetimeTests
         }
     }
 
-    private static void RunSta(Action action)
-    {
-        Exception? error = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { error = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(30)), "The STA test did not complete.");
-        if (error != null)
-            ExceptionDispatchInfo.Capture(error).Throw();
-    }
+    private static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(30), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/AutoNestFormStockOptionsTests.cs

@@ -2,7 +2,6 @@ using System.Collections;
 using System.ComponentModel;
 using System.Globalization;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Text.RegularExpressions;
 using System.Windows.Forms;
 using OpenNest.Forms;
@@ -231,19 +230,6 @@ public class AutoNestFormStockOptionsTests
     private static void SetValue(object row, string property, object value) =>
         TypeDescriptor.GetProperties(row)[property]!.SetValue(row, value);
 
-    private static void RunSta(Action action)
-    {
-        Exception? error = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { error = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(15)), "The STA test did not complete.");
-        if (error != null)
-            ExceptionDispatchInfo.Capture(error).Throw();
-    }
+    private static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(15), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/AutomaticCutOffFormTests.cs

@@ -1,5 +1,4 @@
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.CNC;
 using OpenNest.Controls;
@@ -354,24 +353,6 @@ public class AutomaticCutOffFormTests
         typeof(AutomaticCutOffForm).GetMethod(method, BindingFlags.Instance | BindingFlags.NonPublic)!
             .Invoke(form, arguments);
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try
-            {
-                action();
-            }
-            catch (Exception ex)
-            {
-                failure = ex;
-            }
-        });
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        thread.Join();
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, Timeout.InfiniteTimeSpan, "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/BestFitViewerOperationLifetimeTests.cs

@@ -1,6 +1,5 @@
 using System.Collections.Concurrent;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.CNC;
 using OpenNest.Collections;
@@ -408,19 +407,6 @@ public class BestFitViewerOperationLifetimeTests
     private static void Invoke(BestFitViewerForm form, string name, params object[] args) =>
         typeof(BestFitViewerForm).GetMethod(name, BindingFlags.Instance | BindingFlags.NonPublic)!.Invoke(form, args);
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { failure = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(60)), "STA test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(60), "STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/EditNestFocusTraversalTests.cs

@@ -1,5 +1,4 @@
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.Forms;
 
@@ -146,25 +145,6 @@ public class EditNestFocusTraversalTests
         tabControl.SelectedTab = Assert.IsType<TabPage>(plates.Parent);
     }
 
-    private static void RunSta(System.Action testBody)
-    {
-        Exception? testFailure = null;
-        var staThread = new Thread(() =>
-        {
-            try
-            {
-                testBody();
-            }
-            catch (Exception error)
-            {
-                testFailure = error;
-            }
-        })
-        { IsBackground = true };
-        staThread.SetApartmentState(ApartmentState.STA);
-        staThread.Start();
-        Assert.True(staThread.Join(TimeSpan.FromSeconds(60)), "The focus traversal STA test did not complete.");
-        if (testFailure != null)
-            ExceptionDispatchInfo.Capture(testFailure).Throw();
-    }
+    private static void RunSta(System.Action testBody) =>
+        StaTestThread.Run(testBody, TimeSpan.FromSeconds(60), "The focus traversal STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/LeadInSidePanelTests.cs

@@ -1,4 +1,3 @@
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.Forms;
 
@@ -40,25 +39,6 @@ public class LeadInSidePanelTests
         Assert.Equal("Select", form.PlateView.Status);
     });
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try
-            {
-                action();
-            }
-            catch (Exception ex)
-            {
-                failure = ex;
-            }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(60)), "The STA test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(60), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/NestReportExportTests.cs

@@ -1,5 +1,4 @@
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.CNC;
 using OpenNest.Controls;
@@ -299,25 +298,6 @@ public class NestReportExportTests
         internal override void ReportSuccess(string destination) => Successes.Add(destination);
     }
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try
-            {
-                action();
-            }
-            catch (Exception ex)
-            {
-                failure = ex;
-            }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(90)), "The STA test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(90), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/NestValidationFormTests.cs

@@ -1,4 +1,3 @@
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.Forms;
 
@@ -62,14 +61,6 @@ public class NestValidationFormTests
         });
     }
 
-    private static void RunSta(Action action)
-    {
-        Exception? error = null;
-        var thread = new Thread(() => { try { action(); } catch (Exception ex) { error = ex; } });
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(15)));
-        if (error != null)
-            ExceptionDispatchInfo.Capture(error).Throw();
-    }
+    private static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(15), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/PlateExportTests.cs

@@ -1,7 +1,6 @@
 using System.Drawing;
 using System.Globalization;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using OpenNest.CNC;
 using OpenNest.Data;
 using OpenNest.Forms;
@@ -453,25 +452,6 @@ public class PlateExportTests
         public void Dispose() => Directory.Delete(DirectoryPath, true);
     }
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try
-            {
-                action();
-            }
-            catch (Exception ex)
-            {
-                failure = ex;
-            }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(90)), "The STA test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(90), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/PlateViewFillLifetimeTests.cs

@@ -1,6 +1,5 @@
 using System.Collections.Concurrent;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.CNC;
 using OpenNest.Controls;
@@ -283,25 +282,6 @@ public class PlateViewFillLifetimeTests
         form.GetType().GetMethod(method, BindingFlags.Instance | BindingFlags.NonPublic)!
             .Invoke(form, arguments);
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try
-            {
-                action();
-            }
-            catch (Exception ex)
-            {
-                failure = ex;
-            }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(60)), "The STA test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(60), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/Forms/PostVerificationFormTests.cs

@@ -206,18 +206,6 @@ public class PostVerificationFormTests
         form.GetType().GetMethod(method, BindingFlags.Instance | BindingFlags.NonPublic)!
             .Invoke(form, arguments);
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { failure = ex; }
-        });
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(30)), "STA verification test timed out.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(30), "STA verification test timed out.");
 }

OpenNest.WinForms.Tests/PlateOverlapOverlayTests.cs

@@ -1,7 +1,6 @@
 using System.Collections.Concurrent;
 using System.Drawing;
 using System.Drawing.Drawing2D;
-using System.Runtime.ExceptionServices;
 using System.Windows.Forms;
 using OpenNest.CNC;
 using OpenNest.Controls;
@@ -1183,19 +1182,6 @@ public class PlateOverlapOverlayTests
         return new Part(new Drawing("same name", program), new Vector(x, 0));
     }
 
-    private static void RunSta(System.Action action)
-    {
-        Exception? failure = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { failure = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(60)), "The STA test did not complete.");
-        if (failure != null)
-            ExceptionDispatchInfo.Capture(failure).Throw();
-    }
+    private static void RunSta(System.Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(60), "The STA test did not complete.");
 }

OpenNest.WinForms.Tests/StaTestThread.cs

@@ -0,0 +1,48 @@
+using System.Runtime.ExceptionServices;
+using System.Windows.Forms;
+
+namespace OpenNest.WinForms.Tests;
+
+/// <summary>
+/// Runs a test body on its own STA thread and rethrows its failure on the
+/// calling test thread.
+/// </summary>
+/// <remarks>
+/// By default WinForms catches an exception raised while dispatching a window
+/// message or a BeginInvoke/SynchronizationContext callback and hands it to the
+/// thread's ThreadException handler. With none registered it opens a modal
+/// ThreadExceptionDialog in an interactive session, which blocks the test until
+/// its join timeout, and silently ignores the exception otherwise. Both the
+/// unhandled-exception mode and the handler are per-thread, so they cannot be
+/// configured once for the assembly: each STA thread selects
+/// <see cref="UnhandledExceptionMode.ThrowException"/> before it creates a
+/// window, and the exception then propagates to the code pumping messages
+/// (Show, DoEvents, ShowDialog) and fails the test.
+/// </remarks>
+internal static class StaTestThread
+{
+    public static void Run(Action body, TimeSpan timeout, string timeoutMessage)
+    {
+        Exception? failure = null;
+        var thread = new Thread(() =>
+        {
+            try
+            {
+                Application.SetUnhandledExceptionMode(UnhandledExceptionMode.ThrowException);
+                body();
+            }
+            catch (Exception ex)
+            {
+                failure = ex;
+            }
+        })
+        {
+            IsBackground = true,
+        };
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+        Assert.True(thread.Join(timeout), timeoutMessage);
+        if (failure != null)
+            ExceptionDispatchInfo.Capture(failure).Throw();
+    }
+}

OpenNest.WinForms.Tests/StaTestThreadTests.cs

@@ -0,0 +1,64 @@
+using System.Runtime.ExceptionServices;
+using System.Windows.Forms;
+
+namespace OpenNest.WinForms.Tests;
+
+public class StaTestThreadTests
+{
+    private const string CallbackMessage = "thrown from a queued UI callback";
+
+    [Fact]
+    public void QueuedCallbackExceptionFailsTheTestInsteadOfOpeningADialog()
+    {
+        var error = Assert.Throws<InvalidOperationException>(() =>
+            StaTestThread.Run(
+                ThrowFromQueuedCallback,
+                TimeSpan.FromSeconds(30),
+                "The STA test did not complete."
+            )
+        );
+
+        Assert.Equal(CallbackMessage, error.Message);
+    }
+
+    [Fact]
+    public void WithoutThrowExceptionModeTheCallbackExceptionNeverReachesTheTestBody()
+    {
+        // Control: the default mode routes the same exception to the thread's
+        // ThreadException handler (or, with none, to a modal dialog), so the
+        // test body itself completes without seeing it.
+        Exception? routed = null;
+        Exception? bodyFailure = null;
+        var thread = new Thread(() =>
+        {
+            Application.ThreadException += (_, e) => routed = e.Exception;
+            try
+            {
+                ThrowFromQueuedCallback();
+            }
+            catch (Exception ex)
+            {
+                bodyFailure = ex;
+            }
+        })
+        {
+            IsBackground = true,
+        };
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+
+        Assert.True(thread.Join(TimeSpan.FromSeconds(30)), "The control STA thread did not complete.");
+        if (bodyFailure != null)
+            ExceptionDispatchInfo.Capture(bodyFailure).Throw();
+        Assert.IsType<InvalidOperationException>(routed);
+        Assert.Equal(CallbackMessage, routed!.Message);
+    }
+
+    private static void ThrowFromQueuedCallback()
+    {
+        using var control = new Control();
+        _ = control.Handle;
+        control.BeginInvoke(new Action(() => throw new InvalidOperationException(CallbackMessage)));
+        Application.DoEvents();
+    }
+}

OpenNest.WinForms.Tests/TestAssemblySetup.cs

@@ -1,22 +0,0 @@
-using System.Runtime.CompilerServices;
-using System.Windows.Forms;
-
-namespace OpenNest.WinForms.Tests;
-
-internal static class TestAssemblySetup
-{
-    // SetUnhandledExceptionMode only changes which *default* dialog style WinForms would
-    // show; it still shows one. A registered ThreadException handler is what actually
-    // suppresses it (WinForms always prefers the handler over the default dialog), which
-    // matters here because none of our tests call Application.Run() to install one.
-    // Without this, an exception thrown inside a WndProc callback (e.g. a DataGridView
-    // commit failure during Show()) pops a modal ThreadExceptionDialog that blocks the
-    // desktop until someone dismisses it by hand, instead of failing the test normally.
-    [ModuleInitializer]
-    internal static void Initialize()
-    {
-        Application.SetUnhandledExceptionMode(UnhandledExceptionMode.ThrowException);
-        Application.ThreadException += (_, e) =>
-            Console.Error.WriteLine($"Suppressed ThreadExceptionDialog: {e.Exception}");
-    }
-}