Fix ToolStrip memory leak due to MouseHoverTimer and #4808

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStrip.cs

@@ -4799,9 +4799,16 @@ internal void OnItemAddedInternal(ToolStripItem item)
         }
     }
 
-    internal void OnItemRemovedInternal(ToolStripItem item)
+    internal void OnItemRemovedInternal(ToolStripItem item, ToolStripItemCollection itemCollection)
     {
         KeyboardToolTipStateMachine.Instance.Unhook(item, ToolTip);
+        if (itemCollection == _toolStripItemCollection)
+        {
+            // To prevent memory leaks when item removed from main collection,
+            // we need to remove it from _displayedItems and _overflowItems too.
+            _displayedItems?.Remove(item);
+            _overflowItems?.Remove(item);
+        }
     }
 
     internal override bool AllowsChildrenToShowToolTips()

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStripItemCollection.cs

@@ -384,7 +384,7 @@ private void OnAfterRemove(ToolStripItem item)
 
             if (_owner is not null)
             {
-                _owner.OnItemRemovedInternal(item);
+                _owner.OnItemRemovedInternal(item, this);
 
                 if (!_owner.IsDisposingItems)
                 {

src/System.Windows.Forms/src/System/Windows/Forms/Input/MouseHoverTimer.cs

@@ -6,9 +6,7 @@ namespace System.Windows.Forms;
 internal class MouseHoverTimer : IDisposable
 {
     private readonly Timer _mouseHoverTimer = new();
-
-    // Consider - weak reference?
-    private ToolStripItem? _currentItem;
+    private readonly WeakReference<ToolStripItem?> _currentItem = new(null);
 
     public MouseHoverTimer()
     {
@@ -18,30 +16,26 @@ public MouseHoverTimer()
 
     public void Start(ToolStripItem? item)
     {
-        if (item != _currentItem)
+        _currentItem.TryGetTarget(out var currentItem);
+        if (item != currentItem)
         {
-            Cancel(_currentItem);
+            Cancel();
+            _currentItem.SetTarget(item);
         }
 
-        _currentItem = item;
-        if (_currentItem is not null)
+        if (item is not null)
         {
             _mouseHoverTimer.Enabled = true;
         }
     }
 
-    public void Cancel()
-    {
-        _mouseHoverTimer.Enabled = false;
-        _currentItem = null;
-    }
-
-    /// <summary> cancels if and only if this item was the one that
-    ///  requested the timer
+    /// <summary>
+    /// Cancels if and only if this <paramref name="item"/> was the one that requested the timer.
     /// </summary>
     public void Cancel(ToolStripItem? item)
     {
-        if (item == _currentItem)
+        _currentItem.TryGetTarget(out var currentItem);
+        if (item == currentItem)
         {
             Cancel();
         }
@@ -53,12 +47,18 @@ public void Dispose()
         _mouseHoverTimer.Dispose();
     }
 
+    private void Cancel()
+    {
+        _mouseHoverTimer.Enabled = false;
+        _currentItem.SetTarget(null);
+    }
+
     private void OnTick(object? sender, EventArgs e)
     {
         _mouseHoverTimer.Enabled = false;
-        if (_currentItem is not null && !_currentItem.IsDisposed)
+        if (_currentItem.TryGetTarget(out var currentItem) && !currentItem.IsDisposed)
         {
-            _currentItem.FireEvent(EventArgs.Empty, ToolStripItemEventType.MouseHover);
+            currentItem.FireEvent(EventArgs.Empty, ToolStripItemEventType.MouseHover);
         }
     }
 }

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripTests.cs

@@ -7304,6 +7304,70 @@ public void ToolStrip_GetNextItem_ItemsBackwardExpected(ToolStripLayoutStyle too
         Assert.False(toolStrip.IsHandleCreated);
     }
 
+    [WinFormsFact]
+    public async Task ToolStrip_MouseHoverTimerStartSuccess()
+    {
+        using CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
+        using MenuStrip menuStrip = new();
+        using ToolStripItem toolStripItem = menuStrip.Items.Add("toolStripItem");
+        toolStripItem.MouseHover += (sender, e) => cancellationTokenSource.Cancel();
+        ((MouseHoverTimer)menuStrip.TestAccessor().Dynamic.MouseHoverTimer).Start(toolStripItem);
+        await Assert.ThrowsAsync<TaskCanceledException>(() => Task.Delay(SystemInformation.MouseHoverTime * 2, cancellationTokenSource.Token));
+    }
+
+    [WinFormsFact]
+    public void ToolStrip_MouseHoverTimer_ItemDispose()
+    {
+        WeakReference<ToolStripItem> currentItemWR;
+        using MenuStrip menuStrip = new();
+        MouseHoverTimer mouseHoverTimer = (MouseHoverTimer)menuStrip.TestAccessor().Dynamic.MouseHoverTimer;
+        TimerStartAndItemDispose();
+        GC.Collect();
+        GC.WaitForPendingFinalizers();
+        GC.Collect();
+        GC.WaitForPendingFinalizers();
+        Assert.False(currentItemWR.TryGetTarget(out _));
+
+        void TimerStartAndItemDispose()
+        {
+            using ToolStripItem toolStripItem = menuStrip.Items.Add("toolStripItem");
+            mouseHoverTimer.Start(toolStripItem);
+            currentItemWR = mouseHoverTimer.TestAccessor().Dynamic._currentItem;
+            Assert.True(currentItemWR.TryGetTarget(out _));
+        }
+    }
+
+    [WinFormsFact]
+    public void ToolStrip_displayedItems_Clear()
+    {
+        using ToolStripMenuItem toolStripMenuItem = new(nameof(toolStripMenuItem));
+        using ToolStripMenuItem listToolStripMenuItem = new(nameof(listToolStripMenuItem));
+        toolStripMenuItem.DropDownItems.Add(listToolStripMenuItem);
+        toolStripMenuItem.DropDownOpened += (sender, e) =>
+        {
+            for (int i = 0; i < 4; i++)
+                listToolStripMenuItem.DropDownItems.Add("MenuItem" + i);
+
+            listToolStripMenuItem.DropDown.PerformLayout(); // needed to populate DisplayedItems collection
+        };
+
+        toolStripMenuItem.DropDownClosed += (sender, e) =>
+        {
+            while (listToolStripMenuItem.DropDownItems.Count > 0)
+                listToolStripMenuItem.DropDownItems[listToolStripMenuItem.DropDownItems.Count - 1].Dispose();
+
+            GC.Collect();
+            GC.WaitForPendingFinalizers();
+            GC.Collect();
+            GC.WaitForPendingFinalizers();
+        };
+
+        toolStripMenuItem.ShowDropDown();
+        Assert.Equal(4, listToolStripMenuItem.DropDown.DisplayedItems.Count);
+        toolStripMenuItem.HideDropDown();
+        Assert.Equal(0, listToolStripMenuItem.DropDown.DisplayedItems.Count);
+    }
+
     [WinFormsTheory]
     [InlineData(10, 10)]
     [InlineData(0, 0)]