diff --git a/src/DiffEngineViewer.Windows.Tests/WheelNotchesTests.cs b/src/DiffEngineViewer.Windows.Tests/WheelNotchesTests.cs
new file mode 100644
index 00000000..16532290
--- /dev/null
+++ b/src/DiffEngineViewer.Windows.Tests/WheelNotchesTests.cs
@@ -0,0 +1,83 @@
+/// <summary>
+/// Wheel messages into notches. A wheel click sends a whole one; a precision touchpad sends a
+/// fraction of one per message, many times a second, for as long as the fingers move.
+/// </summary>
+public class WheelNotchesTests
+{
+    [Test]
+    public async Task A_wheel_click_is_a_notch()
+    {
+        var notches = new WheelNotches(120);
+
+        await Assert.That(notches.Add(120)).IsEqualTo(1);
+    }
+
+    /// <summary>
+    /// The reported bug: each message divided on its own is zero notches, so two finger scrolling
+    /// moved nothing at all.
+    /// </summary>
+    [Test]
+    public async Task Small_movements_add_up_to_one()
+    {
+        var notches = new WheelNotches(120);
+
+        var before = new[] { notches.Add(20), notches.Add(20), notches.Add(20), notches.Add(20), notches.Add(20) };
+        var last = notches.Add(20);
+
+        await Assert.That(before).IsEquivalentTo([0, 0, 0, 0, 0]);
+        await Assert.That(last).IsEqualTo(1);
+    }
+
+    [Test]
+    public async Task Small_movements_add_up_the_other_way_too()
+    {
+        var notches = new WheelNotches(120);
+
+        var before = new[] { notches.Add(-40), notches.Add(-40) };
+        var last = notches.Add(-40);
+
+        await Assert.That(before).IsEquivalentTo([0, 0]);
+        await Assert.That(last).IsEqualTo(-1);
+    }
+
+    /// <summary>
+    /// What is left over is kept, rather than each notch starting from nothing.
+    /// </summary>
+    [Test]
+    public async Task Keeps_the_remainder_across_notches()
+    {
+        var notches = new WheelNotches(120);
+
+        notches.Add(100);
+        var first = notches.Add(100);
+        var second = notches.Add(100);
+
+        await Assert.That(first).IsEqualTo(1);
+        // 300 in, one notch out, 60 held. The third message crosses 240
+        await Assert.That(second).IsEqualTo(1);
+    }
+
+    /// <summary>
+    /// A movement back the other way undoes what is held, rather than each direction keeping a
+    /// debt that the next movement has to pay off before anything happens.
+    /// </summary>
+    [Test]
+    public async Task A_movement_back_undoes_what_is_held()
+    {
+        var notches = new WheelNotches(120);
+
+        notches.Add(60);
+        notches.Add(-60);
+        var up = new[] { notches.Add(60), notches.Add(60) };
+
+        await Assert.That(up).IsEquivalentTo([0, 1]);
+    }
+
+    [Test]
+    public async Task A_fast_flick_is_several_notches()
+    {
+        var notches = new WheelNotches(120);
+
+        await Assert.That(notches.Add(360)).IsEqualTo(3);
+    }
+}
diff --git a/src/DiffEngineViewer.Windows/ViewerCanvas.cs b/src/DiffEngineViewer.Windows/ViewerCanvas.cs
index 70fb4fe7..fc8b926a 100644
--- a/src/DiffEngineViewer.Windows/ViewerCanvas.cs
+++ b/src/DiffEngineViewer.Windows/ViewerCanvas.cs
@@ -501,13 +501,15 @@ protected override void OnMouseLeave(EventArgs e)
         tips.Forget(this);
     }
 
+    readonly WheelNotches notches = new(SystemInformation.MouseWheelScrollDelta);
+
     protected override void OnMouseWheel(MouseEventArgs e)
     {
         base.OnMouseWheel(e);
-        var notches = e.Delta / SystemInformation.MouseWheelScrollDelta;
-        if (notches != 0)
+        var scrolled = notches.Add(e.Delta);
+        if (scrolled != 0)
         {
-            Scrolled?.Invoke(notches);
+            Scrolled?.Invoke(scrolled);
         }
     }
 
diff --git a/src/DiffEngineViewer.Windows/WheelNotches.cs b/src/DiffEngineViewer.Windows/WheelNotches.cs
new file mode 100644
index 00000000..03aa3bf1
--- /dev/null
+++ b/src/DiffEngineViewer.Windows/WheelNotches.cs
@@ -0,0 +1,30 @@
+/// <summary>
+/// Turns wheel messages into whole notches, keeping what is left over.
+/// <para>
+/// A wheel click sends 120, but a precision touchpad sends a fraction of that per message - ten to
+/// sixty for an ordinary two finger movement. Dividing each message on its own and dropping the
+/// remainder threw all of those away, so the canvas did not move at all under a touchpad while the
+/// docked scrollbar, which accumulates internally, worked.
+/// </para>
+/// <para>
+/// Its own type for the same reason <see cref="QueueTips" /> is: what matters is an accumulation
+/// across messages, and that can be tested where a window cannot.
+/// </para>
+/// </summary>
+sealed class WheelNotches(int perNotch)
+{
+    int remainder;
+
+    /// <summary>
+    /// The whole notches this message completes, which is usually none of one.
+    /// </summary>
+    public int Add(int delta)
+    {
+        remainder += delta;
+        // Truncates toward zero, so a run of small movements one way accumulates and a movement
+        // back the other way undoes it, rather than each direction keeping a debt of its own.
+        var notches = remainder / perNotch;
+        remainder -= notches * perNotch;
+        return notches;
+    }
+}