这个会在鼠标左击的地方画点，但拖动后窗口会变全白

#include <bits/stdc++.h>
#include <windows.h>

using namespace std;

int wdLeft, wdTop, wdWidth = 800, wdHeight = 500;
HWND wdHWND;

HDC hdc;

void Exit() {
	ReleaseDC(wdHWND, hdc);
	cout << "[System] Exit\n";
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT Msg, WPARAM wParam, LPARAM lParam) {
	if (Msg == WM_PAINT) {
		BitBlt(hdc, 0, 0, wdWidth, wdHeight, NULL, 0, 0, SRCAND);
	} else if (Msg == WM_DESTROY) {
		PostQuitMessage(0), Exit();
	} else if (Msg == WM_CHAR) {
		if (wParam == VK_ESCAPE) {
			DestroyWindow(hwnd), Exit();
		}
	} else if (Msg == WM_MOVING) {
		RECT* _rect = (RECT*) lParam;
		wdLeft = _rect -> left, wdTop = _rect -> top;
	} else if (Msg == WM_SIZING) {
		RECT* _rect = (RECT*) lParam;
		if (!wdLeft && !wdTop) {
			wdLeft = _rect -> left, wdTop = _rect -> top;
		}
		_rect -> left = wdLeft, _rect -> top = wdTop;
		_rect -> right = _rect -> left + wdWidth;
		_rect -> bottom = _rect -> top + wdHeight;
	} else if (Msg == WM_LBUTTONDOWN) {
		short _cursorY = lParam >> 16, _cursorX = lParam - (_cursorY << 16);
		cout << "[Message] LButton down at (" << _cursorX << ", " << _cursorY << ")\n";
		
		//PAINTSTRUCT ps;
		//BeginPaint(hwnd, &ps);
		SetPixel(hdc, _cursorX, _cursorY, RGB(255, 255, 255));
		//EndPaint(hwnd, &ps);
	} else {
		return DefWindowProc(hwnd, Msg, wParam, lParam);
	}
	return 0;
}

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

	memset(&wc, 0, sizeof(wc));
	wc.cbSize = sizeof(WNDCLASSEX);
	wc.lpfnWndProc = WndProc;
	wc.hInstance = hInstance;
	wc.hCursor = LoadCursor(NULL, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
	wc.lpszClassName = "WindowClass";
	wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
	wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);

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

	hwnd = wdHWND = CreateWindowEx(WS_EX_CLIENTEDGE, "WindowClass", "Sb(R) Window", WS_VISIBLE | WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, wdWidth, wdHeight, NULL, NULL, hInstance, NULL);
	if (hwnd == NULL) {
		MessageBox(NULL, "Window Creation Failed!", "Error!", MB_ICONEXCLAMATION | MB_OK);
		return 0;
	}
	cout << "[System] Window created successfully, HWND: " << hwnd << '\n';
	
	hdc = GetDC(hwnd);
	/*for (int i = 0; i <= wdHeight; i += 2) {
		for (int j = 0; j <= wdWidth; j += 2) {
			SetPixel(hdc, j, i, RGB(0, 0, 0));
		}
	}*/
	for (short i = 0; i < 128; i++) {
		for (short j = 0; j < 128; j++) {
			SetPixel(hdc, j, i, RGB(i, j, i + j));
		}
	}
	
	while (GetMessage(&msg, NULL, 0, 0) > 0) {
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}
	return msg.wParam;
}

悬赏一关注，感谢