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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[LeafShi1]

• The WM_WINDOWPOSCHANGING handler unconditionally overrides the dropdown height for every position-change event (as long as SWP_NOSIZE is not set):

   if (pos->cy != height)
   {
       pos->cy = height;
   }
  
  

  This means if the OS intentionally adjusts the dropdown height for legitimate reasons (DPI changes, integral height adjustments, display-edge clamping, or monitor work-area boundaries), the fix will forcefully override it. A safer approach would be to limit the intercept to specific scenarios — e.g., only when the dropdown is initially opening or when items have changed.

• If the dropdown is near the bottom of the screen, the OS may reduce the dropdown height to fit within the work area. The current fix would override that clamped value back to the full calculated height, potentially causing the dropdown to extend off-screen. The fix should respect pos->cy when it's smaller than the calculated height if the dropdown position is near a screen edge.