Combobox в окне с другим фоном. Как убрать белые углы combobox?

#pragma comment(linker,""/manifestdependency:type='win32' \
name='Microsoft.Windows.Common-Controls' version='6.0.0.0' \
processorArchitecture='*' publicKeyToken='6595b64144ccf1df' language='*'"")

DWORD dwStyle = WS_CHILD | CBS_DROPDOWNLIST;
if (m_bVisible) dwStyle |= WS_VISIBLE;
m_hWnd = CreateWindow(WC_COMBOBOX, NULL, dwStyle, m_posX, m_posY, m_width, m_height, m_hParent, (HMENU)m_id, m_hInstance, NULL);

SetWindowLongPtr(mycombobox, GWLP_WNDPROC, (LONG_PTR)MyComboProc);

LRESULT CALLBACK MyComboProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
       WNDPROC OldComboProc = (WNDPROC)GetWindowLongPtr(hwnd, GWLP_USERDATA);
    int rc = CallWindowProc(OldComboProc, hwnd, msg, wParam, lParam);
 
    if (msg == WM_ERASEBKGND || msg == WM_PAINT || msg == WM_NCPAINT)
       {
       RECT r;
       HDC hdc = GetWindowDC(hwnd);     
       GetClientRect(hwnd, &r);
       r.bottom = r.top + combobox_size;
       HRGN hframe = CreateRectRgnIndirect(&r);
       FrameRgn(hdc, hframe, brush, 3, 3);
       DeleteObject(hframe);
       ReleaseDC(hwnd, hdc);
        }
 
    return rc;
}

case WM_CTLCOLOREDIT:
    if ((HWND)lParam == m_hSrcListBox)
    {
        return (LRESULT)m_hBrush;
    }
break;

    case WM_ERASEBKGND:
    {
 
        HDC hdc = (HDC) wParam;
        
    //Здесь рисуется фон
    }
    break;