Handling SDL events inside Win32 window

I have difficulty handling SDL events inside a Win32 window. I made a small program to display magenta background. Basically when the user presses 'q' key, the background color should toggle from magenta to red and vice versa.

sdlwin.c

#include <windows.h>

#include <SDL2/SDL.h>

const int win_width = 500;
const int win_height = 450;

int b = 0;

LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    static SDL_Window *sdlWnd;
    SDL_Surface *surface;

    switch (msg)
    {
    case WM_CREATE:
        if(SDL_Init(SDL_INIT_VIDEO) < 0)
        {
            MessageBox(hWnd, TEXT("SDL cannot be initialized!"), TEXT("Error!"),
                MB_ICONEXCLAMATION | MB_OK);
            PostQuitMessage(0);
        }

        sdlWnd = SDL_CreateWindowFrom(hWnd);

        if (sdlWnd == NULL)
        {
            MessageBox(hWnd, TEXT("SDL window cannot be created!"), TEXT("Error!"),
                MB_ICONEXCLAMATION | MB_OK);
            SDL_Quit();
            PostQuitMessage(0);
        }

        SDL_SetWindowTitle(sdlWnd, "SDL Window");

        break;
    case WM_PAINT:
        surface = SDL_GetWindowSurface(sdlWnd);
        SDL_FillRect(surface, &surface->clip_rect, b ? 0xFFFF00FF : 0xFFFF0000);
        SDL_UpdateWindowSurface(sdlWnd);
        break;
    case WM_CLOSE:
        SDL_DestroyWindow(sdlWnd);
        SDL_Quit();
        DestroyWindow(hWnd);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, msg, wParam, lParam);
    }

    return 0;
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    const TCHAR szClassName[] = TEXT("MyClass");

    WNDCLASS wc;
    HWND hWnd;
    MSG msg;

    wc.style         = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc   = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInstance;
    wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = szClassName;
    wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);

    if (!RegisterClass(&wc))
    {
        MessageBox(NULL, TEXT("Window Registration Failed!"), TEXT("Error!"),
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    hWnd = CreateWindow(szClassName,
        TEXT("SDLWin"),
        WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        CW_USEDEFAULT, CW_USEDEFAULT, win_width, win_height,
        NULL, NULL, hInstance, NULL);

    if (hWnd == NULL)
    {
        MessageBox(NULL, TEXT("Window Creation Failed!"), TEXT("Error!"),
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

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

    SDL_Event e;

    while (GetMessage(&msg, NULL, 0, 0))
    {
        while (SDL_PollEvent(&e) != 0)
        {
            if (e.type == SDL_KEYDOWN && e.key.keysym.sym == SDLK_q)
            {
                b = !b;
                //MessageBox(NULL, TEXT("bar"), TEXT("foo"), MB_ICONEXCLAMATION | MB_OK);
            }
        }

        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return (int) msg.wParam;
}

When I uncomment this line MessageBox(NULL, TEXT("bar"), TEXT("foo"), MB_ICONEXCLAMATION | MB_OK);, the window didn't respond when I press 'q' key but if I add this line, the window background color changed after I close the message box. I'm uncertain why this weird behaviour happens. My question is how to properly handle SDL events inside Win32 window. Any form of help is appreciated.

You need to call InvalidateRect(hWnd, NULL, TRUE/FALSE); after changing b. That will post a WM_Paint message for your Window to redraw.