ToolStripMenuItem can lead to memory leaks

• .NET Core Version: all up to latest (5.0.5)

• Have you experienced this same bug with .NET Framework?: Yes

Problem description: ToolStripMenuItem can lead to memory leaks in some special conditions.

• Menu with submenu.

 windowsToolStripMenuItem 
  -> listToolStripMenuItem

• Populate and clear submenu in runtime. In fact, only cleaning is important.

private void WindowsToolStripMenuItem_DropDownOpened(object sender, EventArgs e)
{
    for (int i = 0; i < 4; i++)
        listToolStripMenuItem.DropDownItems.Add("MenuItem" + i);
}

private void windowsToolStripMenuItem_DropDownClosed(object sender, EventArgs e)
{
    while (listToolStripMenuItem.DropDownItems.Count > 0)
        listToolStripMenuItem.DropDownItems[listToolStripMenuItem.DropDownItems.Count - 1].Dispose();
}

In certain conditions (I think that is - open only main menu and not opening submenu listToolStripMenuItem) after closing main menu some of listToolStripMenuItem subitems will remain in memory. See video below. This happens due to _displayedItems collection still have live references to removed menu items. I think this collection ( and may be _overflowItems too ) must be checked on item removal.

https://user-images.githubusercontent.com/17767561/115139643-acf23080-a03b-11eb-9be9-c0e4c2527f38.mp4

Workaround: You can call listToolStripMenuItem.DropDown.PerformLayout(); after items removal. This lead to call of SetDisplayedItems() which will clear _displayedItems of removed elements.

Expected behavior: All removed and disposed menu elements must be eligible to GC (have no live references).

Minimal repro: WinFormsCoreTest_Grid_Menu.zip