BitBlt每次运行结果很诡异

windows程序设计第14章位图的第一个demo，代码如下

#include <Windows.h>

LRESULT	CALLBACK	WndProc(HWND, UINT, WPARAM, LPARAM);

int	WINAPI	WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
	PSTR szCmdLine, int iCmdShow)
{
	static	TCHAR	szAppName[] = TEXT("BitBlt");

	MSG			msg;
	HWND		hwnd;
	WNDCLASS	wndclass;

	wndclass.style = CS_HREDRAW | CS_VREDRAW;
	wndclass.lpfnWndProc = WndProc;
	wndclass.cbClsExtra = 0;
	wndclass.cbWndExtra = 0;
	wndclass.hInstance = hInstance;
	wndclass.hIcon = LoadIcon(NULL, IDI_INFORMATION);
	wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
	wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
	wndclass.lpszMenuName = NULL;
	wndclass.lpszClassName = szAppName;

	if(!RegisterClass(&wndclass))
	{
		MessageBox(NULL, TEXT("This program requires Windows NT!"),
			szAppName, MB_ICONERROR);
		return 0;
	}

	hwnd = CreateWindow(szAppName, TEXT("BitBlt Demo"),
		WS_OVERLAPPEDWINDOW | WS_CLIPCHILDREN,
		CW_USEDEFAULT, CW_USEDEFAULT,
		CW_USEDEFAULT, CW_USEDEFAULT,
		NULL, NULL, hInstance, NULL);

	ShowWindow(hwnd, iCmdShow);
	UpdateWindow(hwnd);

	while(GetMessage(&msg, NULL, 0, 0))
	{
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}

	return msg.wParam;
}

LRESULT	CALLBACK	WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
	static int cxClient, cyClient, cxSource, cySource;
	HDC	hdcClient, hdcWindow;
	int	x, y;
	PAINTSTRUCT ps;

	switch(message)
	{
	case WM_CREATE:
		cxSource = GetSystemMetrics(SM_CXSIZEFRAME) + GetSystemMetrics(SM_CXSIZE);

		cySource = GetSystemMetrics(SM_CYSIZEFRAME) + GetSystemMetrics(SM_CYSIZE);
		return 0;

	case WM_SIZE:
		cxClient = LOWORD(lParam);
		cyClient = HIWORD(lParam);
		return 0;

	case WM_PAINT:
		hdcClient = BeginPaint(hwnd, &ps);
		hdcWindow = GetWindowDC(hwnd);

		for(y = 0; y < cyClient; y += cySource)
		{
			for(x = 0; x < cxClient; x += cxSource)
			{
				BitBlt(hdcClient, x, y, cxSource, cySource, hdcWindow, 0, 0, SRCCOPY);
			}
		}

		ReleaseDC(hwnd, hdcWindow);
		EndPaint(hwnd, &ps);
		return 0;

	case WM_DESTROY:
		PostQuitMessage(0);
		return 0;
	}

	return DefWindowProc(hwnd, message, wParam, lParam);
}

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