//PD Following is MFC source code. My comments notes with "//PD"
//PD Here's how MFC handles WM_ACTIVATE for frame windows:

void CFrameWnd::OnActivate(UINT nState, CWnd* pWndOther, BOOL bMinimized)
{
    CWnd::OnActivate(nState, pWndOther, bMinimized);

    // get top level frame unless this is a child window
    // determine if window should be active or not
    CFrameWnd* pTopLevel = (GetStyle() & WS_CHILD) ? this : GetTopLevelFrame();
    ASSERT(pTopLevel != NULL);
    CWnd* pActive = (nState == WA_INACTIVE ? pWndOther : this);
    BOOL bStayActive =
        (pTopLevel == pActive ||
        (pTopLevel == pActive->GetTopLevelFrame() &&
        (pActive == pTopLevel ||
            pActive->SendMessage(WM_FLOATSTATUS, FS_SYNCACTIVE) != 0)));
    pTopLevel->m_nFlags &= ~WF_STAYACTIVE;
    if (bStayActive)
        pTopLevel->m_nFlags |= WF_STAYACTIVE;

    // sync floating windows to the new state
    NotifyFloatingWindows(bStayActive ? FS_ACTIVATE : FS_DEACTIVATE);

    // get active view (use active frame if no active view)
    CView* pActiveView = GetActiveView();
    if (pActiveView == NULL)
        pActiveView = GetActiveFrame()->GetActiveView();

    // when frame gets activated, re-activate current view
    if (pActiveView != NULL)
    {
        if (nState != WA_INACTIVE && !bMinimized)
            pActiveView->OnActivateView(TRUE, pActiveView, pActiveView);

        // always notify the view of frame activations
        pActiveView->OnActivateFrame(nState, this);
    }
}

//PD This is the function that causes the bug
//PD
void CFrameWnd::NotifyFloatingWindows(DWORD dwFlags)
{
    ASSERT_VALID(this);
    ASSERT(m_hWnd != NULL);

    // get top level parent frame window first unless this is a child window
    CFrameWnd* pParent = (GetStyle() & WS_CHILD) ? this : GetTopLevelFrame();
    ASSERT(pParent != NULL);
    if (dwFlags & (FS_DEACTIVATE|FS_ACTIVATE))
    {
        // update parent window activation state
        BOOL bActivate = !(dwFlags & FS_DEACTIVATE);
        BOOL bEnabled = pParent->IsWindowEnabled();

        if (bActivate && bEnabled && pParent != this)
        {
            //PD Note special code for Excel -- Sheesh!
            //PD
            // Excel will try to Activate itself when it receives a
            // WM_NCACTIVATE so we need to keep it from doing that here.
            m_nFlags |= WF_KEEPMINIACTIVE;
            pParent->SendMessage(WM_NCACTIVATE, TRUE);
            m_nFlags &= ~WF_KEEPMINIACTIVE;
        }
        else
        {
            //PD this line contains the bug: should not send WM_NCACTIVATE to
            //PD pParent when pParent==this !!
            //PD
            pParent->SendMessage(WM_NCACTIVATE, FALSE);
        }
    }

    // then update the state of all floating windows owned by the parent
    HWND hWnd = ::GetWindow(::GetDesktopWindow(), GW_CHILD);
    while (hWnd != NULL)
    {
        if (AfxIsDescendant(pParent->m_hWnd, hWnd))
            ::SendMessage(hWnd, WM_FLOATSTATUS, dwFlags, 0);
        hWnd = ::GetWindow(hWnd, GW_HWNDNEXT);
    }
}

Figure 2   MFC 2.3 WM_ACTIVATE Handler

//PD Here's the WM_ACTIVATE handler from and older version of MFC (2.53)
//PD

void CFrameWnd::OnActivate(UINT nState, CWnd* pWndOther, BOOL bMinimized)
{
    ASSERT_VALID(GetActiveFrame());

    // get active view (use active frame if no active view)
    CView* pActiveView = GetActiveView();
    if (pActiveView == NULL)
        pActiveView = GetActiveFrame()->GetActiveView();

    if (nState != WA_INACTIVE && !bMinimized)
    {
        // when frame gets activated, re-activate current view
        if (pActiveView != NULL)
            pActiveView->OnActivateView(TRUE, pActiveView, pActiveView);
    }

    // always notify the view of frame activations
    if (pActiveView != NULL)
        pActiveView->OnActivateFrame(nState, this);

    CWnd::OnActivate(nState, pWndOther, bMinimized);
}