ToolStripMenuItem can lead to memory leaks

[kirsan31]

• .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`](https://github.com/dotnet/winforms/blob/c9287e9ca780cd1828633fd3c6e43a6601896147/src/System.Windows.Forms/src/System/Windows/Forms/ToolStrip.cs#L40) 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()`](https://github.com/dotnet/winforms/blob/c9287e9ca780cd1828633fd3c6e43a6601896147/src/System.Windows.Forms/src/System/Windows/Forms/ToolStrip.cs#L4561) 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](https://github.com/dotnet/winforms/files/6330797/WinFormsCoreTest_Grid_Menu.zip)

[Tanya-Solyanik]

@Olina-Zhang - could you please retest this issue?

[Olina-Zhang]

@Olina-Zhang - could you please retest this issue?

Verified this issue in the latest .Net 8.0 build as above sample application and video in VS, have same testing result, so still repro.

[elachlan]

Dispose has a section to dispose of the items. I guess we just do the same for the DisplayedItems and OverflowItems https://github.com/dotnet/winforms/blob/e199c86138cf8fee176bddd3f7e19ec62a526765/src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStrip.cs#L2011-L2020

[elachlan]

We might also want to look at OnItemRemovedInternal. Which is called by ToolStripItemCollection when an item is removed (such as when ToolStripItem.Dispose is called). https://github.com/dotnet/winforms/blob/e199c86138cf8fee176bddd3f7e19ec62a526765/src/System.Windows.Forms/src/System/Windows/Forms/Controls/ToolStrips/ToolStrip.cs#L4906-L4909

We could then trigger a cleanup DisplayedItems and OverflowItems, I think ToolStripItemCollection might need to pass itself to OnItemRemovedInternal so we can cleanup based on which list had the item removed.

[Olina-Zhang]

Verified this issue in the latest .Net 9.0 RC2 SDK build: 9.0.100-rc.2.24468.2 + binaries built fromWinforms repo main branch, it was fixed.

https://github.com/user-attachments/assets/4eab15ae-d2d4-48e3-9e88-a772b698b30d