Fix ComboBox dropdown height after clearing or removing all items

[JayashreeSF3546]

Fixes #339

Proposed changes

• Update ComboBoxChildNativeWindow handling for the dropdown list window.
• Intercept WM_WINDOWPOSCHANGING for the native dropdown LISTBOX and update WINDOWPOS.cy with the managed calculated dropdown height.
• Ensure dropdown height reflects current Items.Count / DropDownHeight even after Items.Clear() or removing the last item.

Customer Impact

• Fixes incorrect ComboBox dropdown height after items are cleared or removed at runtime.
• Prevents the dropdown from using a stale native cached height when the item count becomes zero.

Regression?

• No

Risk

• Low.
• The change is limited to the ComboBox dropdown LISTBOX window during WM_WINDOWPOSCHANGING.
• The fix only updates height when the native window position change includes sizing and does not affect other child window types.
• Existing IntegralHeight behavior is preserved while ensuring the final committed dropdown height matches WinForms calculated height.

Screenshots

Before

After

Test methodology

• Added/updated unit test coverage to verify WM_WINDOWPOSCHANGING updates WINDOWPOS.cy to the managed calculated dropdown height.
• Verified the dropdown height is corrected after Items.Clear().
• Verified the fix does not update height when SWP_NOSIZE is set.

Accessibility testing

• No accessibility behavior change expected.
• The change only corrects the native dropdown LISTBOX height calculation.
• No changes were made to accessible names, roles, states, keyboard navigation, or UI Automation patterns.

Test environment(s)

• Windows 11
• .NET SDK: 11.0.100-preview.3.26170.106

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR addresses a long-standing WinForms ComboBox issue where the dropdown LISTBOX can keep using a stale cached height after all items are cleared/removed, by enforcing the managed dropdown height during native sizing.

Changes:

• Refactors dropdown height computation into a reusable helper (GetCalculatedDropDownHeight).
• Intercepts WM_WINDOWPOSCHANGING for the dropdown LISTBOX native window and rewrites WINDOWPOS.cy to the managed-calculated height when sizing occurs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs	Extracts dropdown height calculation into GetCalculatedDropDownHeight and reuses it from UpdateDropDownHeight.
src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ComboBoxChildNativeWindow.cs	Handles WM_WINDOWPOSCHANGING for the dropdown LISTBOX to enforce managed height before the OS commits the size.

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