Интересный вопрос по поводу связи таймера с WM_PAINT

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

LRESULT CALLBACK WindowProc(HWND, UINT, WPARAM, LPARAM);
void CALLBACK TimerProc(HWND hWnd, UINT msg, UINT idTimer, DWORD dwTime);
LPCSTR szAppName  = "L";
HINSTANCE hInst;

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine, int nCmdShow){
    HWND hWnd;
    MSG msg;
    WNDCLASS wc;

    hInst = hInstance;

    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = WindowProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hCursor = LoadCursor(NULL,IDC_ARROW);
    wc.hIcon = LoadIcon(NULL,IDI_APPLICATION);
    wc.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
    wc.lpszClassName = szAppName;
    wc.lpszMenuName = NULL;
    RegisterClass(&wc);
    hWnd = CreateWindow(szAppName,
        "L",
        WS_POPUPWINDOW ,//WS_OVERLAPPEDWINDOW,
        40, /* X-Position auf dem Monitor */
        40, /* Y-Position auf dem Monitor */
        300, /* Fensterbreite */
        300, /* Fensterhoehe */
        NULL,
        NULL,
        hInstance,
        NULL);

    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);
    while(1) {
            while(GetMessage(&msg,hWnd,0,0) == TRUE)
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
        Sleep(50);
    }
    return 0;
}

LRESULT WINAPI WindowProc(HWND hWnd,UINT message,WPARAM wParam,LPARAM lParam)
{

    switch(message)
    {
    case WM_LBUTTONDOWN:
        SetTimer(hWnd, 1, 1000, &TimerProc);
        break;

    case WM_PAINT:
        HDC hDC;

        hDC = GetDC(hWnd);
        TextOut(hDC, 5, 5, "HELLO WORLD", sizeof("HELLO WORLD"));
        ReleaseDC(hWnd, hDC);
        DeleteDC(hDC);
        break;

    case WM_DESTROY:
        PostQuitMessage(0);
        exit(0);
        break;
    default: return(DefWindowProc(hWnd,message,wParam,lParam));
        break;
    }
    return(0L);

}

void CALLBACK TimerProc(HWND hWnd, UINT msg, UINT idTimer, DWORD dwTime) {
    MessageBox (hWnd,"hello", "!!!", MB_OK);
    KillTimer(hWnd, 1);
}

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

LRESULT CALLBACK WindowProc(HWND, UINT, WPARAM, LPARAM);
void CALLBACK TimerProc(HWND hWnd, UINT msg, UINT idTimer, DWORD dwTime);
LPCSTR szAppName  = "L";
HINSTANCE hInst;

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine, int nCmdShow){
    HWND hWnd;
    MSG msg;
    WNDCLASS wc;

    hInst = hInstance;

    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = WindowProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hCursor = LoadCursor(NULL,IDC_ARROW);
    wc.hIcon = LoadIcon(NULL,IDI_APPLICATION);
    wc.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
    wc.lpszClassName = szAppName;
    wc.lpszMenuName = NULL;
    RegisterClass(&wc);
    hWnd = CreateWindow(szAppName,
        "L",
        WS_POPUPWINDOW ,//WS_OVERLAPPEDWINDOW,
        40, /* X-Position auf dem Monitor */
        40, /* Y-Position auf dem Monitor */
        300, /* Fensterbreite */
        300, /* Fensterhoehe */
        NULL,
        NULL,
        hInstance,
        NULL);

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

    while(1) {
            while(GetMessage(&msg,hWnd,0,0) == TRUE)
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
        Sleep(50);
    }
    return 0;
}

LRESULT WINAPI WindowProc(HWND hWnd,UINT message,WPARAM wParam,LPARAM lParam)
{
    switch(message)
    {
    case WM_LBUTTONDOWN:
        SetTimer(hWnd, 1, 1000, &TimerProc);
        break;
    case WM_PAINT:
        HDC hDC;
        PAINTSTRUCT PaintStruct;

        hDC = BeginPaint(hWnd,&PaintStruct);
        TextOut(hDC, 5, 5, "HELLO WORLD", sizeof("HELLO WORLD"));
        EndPaint(hWnd,&PaintStruct);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        exit(0);
        break;
    default: return(DefWindowProc(hWnd,message,wParam,lParam));
        break;
    }
    return(0L);

}

void CALLBACK TimerProc(HWND hWnd, UINT msg, UINT idTimer, DWORD dwTime) {
    MessageBox (hWnd,"hello", "!!!", MB_OK);
    KillTimer(hWnd, 1);
}