MFC: how to keep child CWnd dialog from jumping upon resizing the parent window?

I'm writing an MFC dialog with multiple controls. I have currently have a CWnd that is placed on the right half of the dialog. Upon clicking an edit button, the child CWnd is resized to take up a larger portion of the dialog.

However, now when I try to resize the window, the child CWnd jumps back to where it was originally. I cannot seem to figure out how to keep it in it's new position when resizing.

Relevant code:

OnInit() {
    //the grouper rectangle
    CRect rectHTMLGrouper;
    m_grpHTMLbox.GetWindowRect(&rectHTMLGrouper);
    ScreenToClient(&rectHTMLGrouper);

    //the new rectangle to use for positioning
    CRect rectHtml;
    rectHtml.left = rectHTMLGrouper.left + PREVIEW_EDITOR_LEFT;
    rectHtml.right = rectHTMLGrouper.right - PREVIEW_EDITOR_RIGHT;
    rectHtml.top = rectHTMLGrouper.top + PREVIEW_EDITOR_TOP;
    rectHtml.bottom = rectHTMLGrouper.bottom - PREVIEW_EDITOR_BOTTOM;    

    //this inits my editor and sets the position 
    m_wHtmlEditor.CreateHtmlEditor(rectHTMLGrouper, this, WS_CHILD | WS_VISIBLE | WS_CLIPCHILDREN);

    //CodeJock - XTREMEToolkit Call for SetResize Logic
    SetResize(m_wHtmlEditor.GetDlgCtrlID(), LEFT_PANE_RESIZE, 0, 1, 1);
    m_wHtmlEditor.SetWindowPos(&CWnd::wndTop, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOACTIVATE | SWP_NOMOVE);
}


OnEditMode() {

    //enlarge the editor to take up the full dialog
    CRect parentClientRect;
    m_wHtmlEditor.GetParent()->GetClientRect(&parentClientRect);
    m_wHtmlEditor.SetWindowPos(&CWnd::wndTop, parentClientRect.left + edgePadding, parentClientRect.top + editorTopPadding, parentClientRect.right - (edgePadding * 2), parentClientRect.bottom - bottomPadding, SWP_NOOWNERZORDER);

    return;
}

Upon clicking an edit button, the child CWnd is resized to take up a larger portion of the dialog.

You have to handle that same resize in your OnSize() (ON_WM_SIZE()) message handler (using some kind of BOOL member to keep track of the child window's status).

OnSize() is called repeatedly while resizing the dialog.

Example:

// .h
BOOL m_bIsEditMode;

// .cpp
// keep track of m_bIsEditMode

void CMyDlg::OnSize(UINT nType, int cx, int cy)
{
    CDialog::OnSize(nType, cx, cy);

    if (m_bIsEditMode) {

        //enlarge the editor to take up the full dialog
        m_wHtmlEditor.MoveWindow (0, 0, cx, cy);
    }
}