Why is the WM_ERASEBKGND message sent when I reduce the window size?

Question

I noticed that when I reduce the size of the window, a message is sent WM_ERASEBKGND, can anyone explain why this message is sent in this case, I mean, I understand that when I increase the size of the window, I have to remove the background of the new displayed area but why do I need to erase the background when I reduce the window size ?!

This is a screenshot that illustrates this behavior (I installed Spy ++ only for displaying messages WM_ERASEBKGND):

This is the code I used to create the window:

#include <Windows.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch(message)
    {
    case WM_CLOSE:
        DestroyWindow(hWnd);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wc;
    wc.cbSize = sizeof(WNDCLASSEX);
    wc.style = 0;
    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_BTNFACE+1);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = "WinClass";
    wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
    RegisterClassEx(&wc);

    HWND hWnd = CreateWindowEx(0, "WinClass", "My Window", WS_OVERLAPPEDWINDOW, 400, 400, 400, 400, NULL, NULL, hInstance, NULL);
    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);

    MSG msg;
    while(GetMessage(&msg, NULL, 0, 0) > 0)
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}

---

Edit:

I forgot to mention an important detail: when I reduce the size of the window, a message is sent without a message .WM_PAINT

Answer 1

, , , CS_HREDRAW / CS_VREDRAW . .

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