Fix Win32Exception when clicking ToolBar Options after narrowing form width

[Copilot]

Fixes #14077

Proposed changes

• Modified ToolStripControlHost.SyncControlParent() to keep hosted controls parented to Owner ToolStrip when ParentInternal is a ToolStripDropDown
• Added unit tests verifying hosted controls don't get added to dropdown's Controls collection

Root cause: When ToolStripControlHost items (e.g., ToolStripComboBox, ToolStripProgressBar) move to overflow, SyncControlParent() was adding the hosted control to the ToolStripOverflow's Controls collection. When the dropdown shows, CreateControl calls SetParentHandle which fails because ToolStripDropDown is a TopLevel popup window (DPI awareness/window hierarchy constraints).

// Before: always used ParentInternal
ReadOnlyControlCollection? newControls = GetControlCollection(ParentInternal);

// After: use Owner when ParentInternal is a dropdown
ToolStrip? targetParent = ParentInternal is ToolStripDropDown ? Owner : ParentInternal;
ReadOnlyControlCollection? newControls = GetControlCollection(targetParent);

Customer Impact

• Unhandled Win32Exception crashes app when clicking overflow button on narrowed ToolStrip containing control hosts
• Blocking issue for apps using ToolStripComboBox or ToolStripProgressBar in constrained layouts

Regression?

• No - never worked in .NET Core/.NET 5+; worked in .NET Framework

Risk

• Low - targeted fix affecting only ToolStripControlHost parenting when parent is a dropdown

Test methodology

• Added unit tests for dropdown parenting scenarios
• Verified hosted control stays in Owner's Controls collection when Parent is ToolStripDropDown

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>After reducing the form width, clicking the ToolStrip options triggers a Win32Exception error</issue_title>
<issue_description>### .NET version

.NET SDK 10.0.00-rtm.25523.111

Did it work in .NET Framework?

Yes

Did it work in any of the earlier releases of .NET Core or .NET 5+?

No, it still repro on .NET 8.0 and .NET 9.0

Issue description

When the form width is reduced to prevent Toolstrip items from displaying, clicking the ToolBar Options throws a System.ComponentModel.Win32Exception (0x80004005): Failed to set Win32 parent window of the Control error.

.NET project

.NET framework project

Call Stack

See the end of this message for details on invoking 
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.ComponentModel.Win32Exception (0x80004005): Failed to set Win32 parent window of the Control.
   at System.Windows.Forms.Control.SetParentHandle(HWND value)
   at System.Windows.Forms.Control.CreateControl(Boolean ignoreVisible)
   at System.Windows.Forms.Control.CreateControl()
   at System.Windows.Forms.Control.WmShowWindow(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ToolStrip.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(HWND hWnd, UInt32 msg, WPARAM wparam, LPARAM lparam)

Steps to reproduce

1. Create a WinForms .NET project
2. Add a ToolStrip to form designer
3. Add toolStripComboBox and toolStripProgressBar to ToolStrip control
4. Ctrl + F5 to run this project
5. Narrow down the width of form to make sure no toolstrip Item display on form designer
6. Click on the ToolBar Options</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes After reducing the form width, clicking the ToolStrip options triggers a Win32Exception error #14094

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