GetWindowTheme looses HTHEME after SetWindowLongPtr call

// TestGetWindowTheme.cpp : Defines the entry point for the application.
//

#include "stdafx.h"
#include "TestGetWindowTheme.h"

#include <assert.h>
#include <uxtheme.h>
#pragma comment(lib, "UxTheme.lib")



#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
WCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
WCHAR szWindowClass[MAX_LOADSTRING];            // the main window class name

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);
LRESULT CALLBACK OriginalWindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);


int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.

    // Initialize global strings
    LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadStringW(hInstance, IDC_TESTGETWINDOWTHEME, szWindowClass, MAX_LOADSTRING);
    if(!MyRegisterClass(hInstance))
    {
        return -1;
    }

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_TESTGETWINDOWTHEME));

    MSG msg;

    // Main message loop:
    while (GetMessage(&msg, nullptr, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}



//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEXW wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = sizeof(HWND);
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_TESTGETWINDOWTHEME));
    wcex.hCursor        = LoadCursor(nullptr, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = MAKEINTRESOURCEW(IDC_TESTGETWINDOWTHEME);
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    BOOL bRes = RegisterClassExW(&wcex);



    WNDCLASSEXW wc = {0};

    wc.cbSize = sizeof(wc);
    wc.lpszClassName = L"myclassname";
    wc.style = CS_DBLCLKS | CS_PARENTDC;
    wc.lpfnWndProc = (WNDPROC)OriginalWindowProc;
    wc.hInstance = hInstance;

    if(!::RegisterClassExW(&wc))
    {
        int Err = ::GetLastError();
        bRes = FALSE;
        assert(NULL);
    }

    return bRes;
}

//
//   FUNCTION: InitInstance(HINSTANCE, int)
//
//   PURPOSE: Saves instance handle and creates main window
//
//   COMMENTS:
//
//        In this function, we save the instance handle in a global variable and
//        create and display the main program window.
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   hInst = hInstance; // Store instance handle in our global variable

   HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, hInstance);

   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
//  FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  PURPOSE: Processes messages for the main window.
//
//  WM_COMMAND  - process the application menu
//  WM_PAINT    - Paint the main window
//  WM_DESTROY  - post a quit message and return
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_CREATE:
        {
            int nErr;

            HWND hChWnd = ::CreateWindowEx(0,
                L"myclassname",
                L"",
                WS_VISIBLE | WS_CHILD,
                50, 20, 100, 100, hWnd, NULL, hInst, NULL);
            if(!hChWnd)
            {
                nErr = ::GetLastError();
                assert(NULL);
            }

            ::SetLastError(0);
            ::SetWindowLongPtr(hWnd, 0, (LONG_PTR)hChWnd);
            assert(!(nErr = ::GetLastError()));
        }
        break;

    case WM_COMMAND:
        {
            int wmId = LOWORD(wParam);
            // Parse the menu selections:
            switch (wmId)
            {
            case IDM_ABOUT:
                DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
                break;
            case IDM_EXIT:
                DestroyWindow(hWnd);
                break;
            default:
                return DefWindowProc(hWnd, message, wParam, lParam);
            }
        }
        break;

    case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hWnd, &ps);
            // TODO: Add any drawing code that uses hdc here...

            HWND hChildWnd = (HWND)::GetWindowLongPtr(hWnd, 0);
            assert(hChildWnd != 0);

            WCHAR buff[256];
            wsprintfW(buff, L"Click here to illustrate the bug ... hTheme: 0x%p", ::GetWindowTheme(hChildWnd));
            ::TextOut(hdc, 20, 130, buff, (int)wcslen(buff));

            EndPaint(hWnd, &ps);
        }
        break;

    case WM_LBUTTONDOWN:
    {
        HWND hChildWnd = (HWND)::GetWindowLongPtr(hWnd, 0);
        assert(hChildWnd != 0);

        DWORD_PTR dwStyle = ::GetWindowLongPtr(hChildWnd, GWL_STYLE);
    //  dwStyle ^= WS_BORDER;
        ::SetWindowLongPtr(hChildWnd, GWL_STYLE, dwStyle);      //Calling this causes 'hChildWnd' wnd to loose its theme. Why???

        //Redraw self
        ::InvalidateRect(hWnd, NULL, TRUE);
    }
    break;

    case WM_DESTROY:
    {
        HWND hChildWnd = (HWND)::GetWindowLongPtr(hWnd, 0);
        assert(hChildWnd != 0);
        assert(::DestroyWindow(hChildWnd));

        PostQuitMessage(0);
    }
    break;

    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}




LRESULT CALLBACK OriginalWindowProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_NCCREATE:
        {
            HTHEME hTheme = ::OpenThemeData(hWnd, L"EDIT");
            assert(hTheme);
            assert(::GetWindowTheme(hWnd) != 0);

            return 1;
        }
        break;

        case WM_ERASEBKGND:
        {
            HDC hDc = (HDC)wParam;
            RECT rc;
            ::GetClientRect(hWnd, &rc);
            ::FillRect(hDc, &rc, (HBRUSH)::GetStockObject(LTGRAY_BRUSH));
        }
        return 1;

        case WM_DESTROY:
        {
            HTHEME hTheme = ::GetWindowTheme(hWnd);
            assert(hTheme);
            assert(::CloseThemeData(hTheme) == S_OK);
        }
        break;

        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }

    return 0;
}