Clear the queue tooltip when it is forgotten

src/DiffEngineViewer.Windows.Tests/QueueTipsTests.cs

@@ -68,8 +68,43 @@ public async Task ForgettingLetsTheSameRowChange()
         using var tips = new QueueTips();
 
         tips.Apply(owner, 3, "before");
-        tips.Forget();
+        tips.Forget(owner);
         tips.Apply(owner, 3, null);
         await Assert.That(tips.Current(owner)).IsEmpty();
     }
+
+    /// <summary>
+    /// The reported bug reached the other way round: the cursor leaves the queue column for the
+    /// diff panes, which is a Forget - from the mouse leaving, or from the screen being redrawn -
+    /// followed by a row of -1.
+    /// <para>
+    /// Both set the row to -1, so Apply had nothing to do and returned, and the caption for the
+    /// row last hovered stayed registered on the whole canvas. Resting anywhere brought it back,
+    /// over the panes.
+    /// </para>
+    /// </summary>
+    [Test]
+    public async Task ForgettingBeforeLeavingTheColumnLeavesNothingRegistered()
+    {
+        using var owner = new Control();
+        using var tips = new QueueTips();
+
+        tips.Apply(owner, 2, "SolutionA/Tests/ATests.cs:6");
+        tips.Forget(owner);
+        tips.Apply(owner, -1, null);
+
+        await Assert.That(tips.Current(owner)).IsEmpty();
+    }
+
+    [Test]
+    public async Task ForgettingClearsTheText()
+    {
+        using var owner = new Control();
+        using var tips = new QueueTips();
+
+        tips.Apply(owner, 2, "SolutionA/Tests/ATests.cs:6");
+        tips.Forget(owner);
+
+        await Assert.That(tips.Current(owner)).IsEmpty();
+    }
 }

src/DiffEngineViewer.Windows/QueueTips.cs

@@ -58,9 +58,19 @@ public void Apply(Control owner, int queueRow, string? text)
     /// <summary>
     /// Forgets which row is described, so the next move re-applies. For a new screen, which
     /// renumbers the rows under a cursor that has not moved, and for the cursor leaving.
+    /// <para>
+    /// The caption goes with it. Resetting only the row left the last row's text registered on the
+    /// whole canvas, and <see cref="Apply" />'s own early return then kept it there: a cursor on
+    /// the canvas but not on a queue row arrives as row -1, which is the row this just set. So the
+    /// tip popped up over the diff panes, which is the thing this type exists to prevent.
+    /// </para>
     /// </summary>
-    public void Forget() =>
+    public void Forget(Control owner)
+    {
         row = -1;
+        tip.Hide(owner);
+        tip.SetToolTip(owner, null);
+    }
 
     /// <summary>
     /// What would be shown, which is what the tests assert on: the bug this type exists to prevent

src/DiffEngineViewer.Windows/ViewerCanvas.cs

@@ -102,7 +102,7 @@ public void Draw(Screen value)
     {
         screen = value;
         // A new screen renumbers the rows, so a kept index would describe a different entry.
-        tips.Forget();
+        tips.Forget(this);
         Invalidate();
     }
 
@@ -498,7 +498,7 @@ protected override void OnMouseLeave(EventArgs e)
             Cursor = Cursors.Default;
         }
 
-        tips.Forget();
+        tips.Forget(this);
     }
 
     protected override void OnMouseWheel(MouseEventArgs e)