Fix FailFast crash in LabelEditNativeWindow due to garbage collected WinEvent hook delegate

[LeafShi1]

Fixes #14657

Root Cause

Window destroyed → hooks not unregistered → object garbage collected → hook callback invokes freed memory → crash (FailFast).

The original code skipped cleanup when handle was gone, leaving orphaned hooks running.

Proposed changes

This prevents callbacks from invoking garbage-collected delegates.

• Added UnhookWinEventHooks() method to centralize hook cleanup
• Call unhook when handle is destroyed in OnHandleChange()
• Added _isReleasing flag to prevent reentry during cleanup
• Set _winEventProcCallback = null after unhooking to allow GC

Customer Impact

• Applications using screen readers (e.g., Narrator) no longer crash when editing PropertyGrid or TreeView with accessibility enabled.

Regression?

• No

Risk

• Minimal: only defensive guards added, no behavior changes.

Screenshots

Before

protected override void OnHandleChange()
{
    base.OnHandleChange();
    
    if (!IsHandleCreated)
    {
        return;  // ← Hooks remain registered even after window destruction
    }
    
    InstallWinEventHooks();
}

When native window is destroyed, WinEvent hooks are not unregistered. This orphaned hook persists after the managed object is garbage collected, causing late callbacks to invoke freed delegates → FailFast crash.

After

protected override void OnHandleChange()
{
    base.OnHandleChange();
    
    if (!IsHandleCreated)
    {
        UnhookWinEventHooks();  // ← Properly clean up hooks
        return;
    }
    
    InstallWinEventHooks();
}

private void UnhookWinEventHooks()
{
    if (_winEventHooksInstalled)
    {
        PInvoke.UnhookWinEvent(_valueChangeHook);
        PInvoke.UnhookWinEvent(_textSelectionChangedHook);
        _winEventHooksInstalled = false;
        _winEventProcCallback = null;  // Allow GC
    }
}

Hooks are properly unregistered when the native window handle is destroyed, preventing orphaned callbacks and GC-related crashes.

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-preview.6.26310.110

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes a GC-induced FailFast crash in LabelEditNativeWindow accessibility scenarios by ensuring WinEvent hooks are reliably unregistered when the underlying native handle is destroyed, preventing callbacks from invoking a collected delegate.

Changes:

• Centralized WinEvent hook cleanup into UnhookWinEventHooks() and invoked it when the handle is destroyed (OnHandleChange when !IsHandleCreated).
• Added a _isReleasing reentrancy guard to prevent hook install/uninstall cycles during teardown.
• Added callback-side guards and cleared the stored delegate reference after unhooking to avoid stale callbacks and enable GC.

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