diff --git a/OpenNest.WinForms.Tests/Controls/SeparatorPenLifetimeTests.cs b/OpenNest.WinForms.Tests/Controls/SeparatorPenLifetimeTests.cs
new file mode 100644
index 0000000..10ef704
--- /dev/null
+++ b/OpenNest.WinForms.Tests/Controls/SeparatorPenLifetimeTests.cs
@@ -0,0 +1,126 @@
+using System.Drawing;
+using System.Reflection;
+using System.Runtime.ExceptionServices;
+using System.Windows.Forms;
+using OpenNest.Controls;
+
+namespace OpenNest.WinForms.Tests.Controls;
+
+public class SeparatorPenLifetimeTests
+{
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void DisposeReleasesBothPensAndDisposesTheControlOnlyOnce(bool horizontal) => RunSta(() =>
+    {
+        using var control = CreateSeparator(horizontal);
+        using var child = new Control();
+        control.Controls.Add(child);
+        _ = control.Handle;
+        var lightPen = GetPen(control, "lightPen");
+        var darkPen = GetPen(control, "darkPen");
+        using var bitmap = new Bitmap(4, 4);
+        using var graphics = Graphics.FromImage(bitmap);
+        graphics.DrawLine(lightPen, 0, 0, 3, 0);
+        graphics.DrawLine(darkPen, 0, 1, 3, 1);
+        var disposedEvents = 0;
+        control.Disposed += (_, _) => disposedEvents++;
+
+        control.Dispose();
+
+        Assert.True(control.IsDisposed);
+        Assert.True(child.IsDisposed);
+        Assert.False(control.IsHandleCreated);
+        Assert.Equal(1, disposedEvents);
+        Assert.Throws<ArgumentException>(() => graphics.DrawLine(lightPen, 0, 0, 3, 0));
+        Assert.Throws<ArgumentException>(() => graphics.DrawLine(darkPen, 0, 1, 3, 1));
+
+        control.Dispose();
+
+        Assert.True(control.IsDisposed);
+        Assert.Equal(1, disposedEvents);
+        Assert.Throws<ArgumentException>(() => graphics.DrawLine(lightPen, 0, 0, 3, 0));
+        Assert.Throws<ArgumentException>(() => graphics.DrawLine(darkPen, 0, 1, 3, 1));
+    });
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void PaintingKeepsOrientationColorsStylesAndResizeBehavior(bool horizontal) => RunSta(() =>
+    {
+        using var control = CreateSeparator(horizontal);
+        Assert.True(GetStyle(control, ControlStyles.OptimizedDoubleBuffer));
+        Assert.False(GetStyle(control, ControlStyles.Selectable));
+        control.Size = new Size(40, 20);
+        _ = control.Handle;
+        var paintEvents = 0;
+        var resizeEvents = 0;
+        var invalidatedEvents = 0;
+        control.Paint += (_, _) => paintEvents++;
+        control.Resize += (_, _) => resizeEvents++;
+        control.Invalidated += (_, _) => invalidatedEvents++;
+
+        AssertPainting(control, horizontal);
+        Assert.Equal(1, paintEvents);
+
+        control.Size = new Size(30, 40);
+
+        Assert.Equal(1, resizeEvents);
+        Assert.True(invalidatedEvents > 0);
+        AssertPainting(control, horizontal);
+        Assert.Equal(2, paintEvents);
+    });
+
+    private static Control CreateSeparator(bool horizontal) =>
+        horizontal ? new HorizontalLine() : new VerticalLine();
+
+    private static Pen GetPen(Control control, string name) =>
+        Assert.IsType<Pen>(control.GetType()
+            .GetField(name, BindingFlags.Instance | BindingFlags.NonPublic)!.GetValue(control));
+
+    private static bool GetStyle(Control control, ControlStyles style) =>
+        Assert.IsType<bool>(typeof(Control)
+            .GetMethod("GetStyle", BindingFlags.Instance | BindingFlags.NonPublic)!.Invoke(control, [style]));
+
+    private static void AssertPainting(Control control, bool horizontal)
+    {
+        using var bitmap = new Bitmap(control.Width, control.Height);
+        using var graphics = Graphics.FromImage(bitmap);
+        var background = Color.Magenta;
+        graphics.Clear(background);
+        using var args = new PaintEventArgs(graphics, control.ClientRectangle);
+        control.GetType().GetMethod("OnPaint", BindingFlags.Instance | BindingFlags.NonPublic)!
+            .Invoke(control, [args]);
+
+        var midpoint = (horizontal ? control.Height : control.Width) / 2;
+        for (var y = 0; y < bitmap.Height; y++)
+        {
+            for (var x = 0; x < bitmap.Width; x++)
+            {
+                var position = horizontal ? y : x;
+                var expected = background;
+                if (position == midpoint)
+                    expected = ProfessionalColors.SeparatorDark;
+                else if (position == midpoint + 1)
+                    expected = ProfessionalColors.SeparatorLight;
+                Assert.Equal(expected.ToArgb(), bitmap.GetPixel(x, y).ToArgb());
+            }
+        }
+    }
+
+    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();
+    }
+}
diff --git a/OpenNest/Controls/HorizontalLine.cs b/OpenNest/Controls/HorizontalLine.cs
index e97c840..c8d5638 100644
--- a/OpenNest/Controls/HorizontalLine.cs
+++ b/OpenNest/Controls/HorizontalLine.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Drawing;
 using System.Windows.Forms;
 
@@ -18,6 +18,22 @@ namespace OpenNest.Controls
             darkPen = new Pen(ProfessionalColors.SeparatorDark);
         }
 
+        protected override void Dispose(bool disposing)
+        {
+            try
+            {
+                if (disposing)
+                {
+                    lightPen.Dispose();
+                    darkPen.Dispose();
+                }
+            }
+            finally
+            {
+                base.Dispose(disposing);
+            }
+        }
+
         protected override void OnResize(EventArgs e)
         {
             base.OnResize(e);
diff --git a/OpenNest/Controls/VerticalLine.cs b/OpenNest/Controls/VerticalLine.cs
index 449fe70..a0c23a5 100644
--- a/OpenNest/Controls/VerticalLine.cs
+++ b/OpenNest/Controls/VerticalLine.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Drawing;
 using System.Windows.Forms;
 
@@ -18,6 +18,22 @@ namespace OpenNest.Controls
             darkPen = new Pen(ProfessionalColors.SeparatorDark);
         }
 
+        protected override void Dispose(bool disposing)
+        {
+            try
+            {
+                if (disposing)
+                {
+                    lightPen.Dispose();
+                    darkPen.Dispose();
+                }
+            }
+            finally
+            {
+                base.Dispose(disposing);
+            }
+        }
+
         protected override void OnResize(EventArgs e)
         {
             base.OnResize(e);