Setting a child window under Progman window to show video ender desktop icon

Question

I'm trying to make a function similar to the wallpaper engine, by adding a child window to the Progman window. The function did allow me to add the video, however, the window size is fixed, which made me unable to fit the video to the entire desktop.

I tried to change the window size, but neither MoveWindow nor SetWindowPos worked. The Progman's hwnd Rect is just screen size (1920x1080), so it's not a boundary issue. The video window size is 640x480. I'm able to change the location of the window, meaning no permission issue, just can't change the size of it.

Here is the code.

LPCWSTR lpParam = L" videoTest2.mp4 -noborder -fs -loop 0";
STARTUPINFO si{ 0 };
PROCESS_INFORMATION pi{ 0 };

if (CreateProcess(L"ffplay.exe", (LPWSTR)lpParam, 0, 0, 0, 0, 0, 0, &si, &pi)) {
    Sleep(200);

    HWND hProgram = FindWindow(_T("Progman"), _T("Program Manager"));
    HWND hFfplay = FindWindow(L"SDL_app", 0);
    SetParent(hFfplay, hProgram);

    // MoveWindow(hFfplay, 0, 0, 3840, 1080, true);
    SetWindowPos(hFfplay, NULL, 0, 0, 3840, 1080, SWP_NOZORDER);
    ShowWindow(hFfplay, SW_MAXIMIZE);


    EnumWindows(EnumWindowProc, (LPARAM)SW_HIDE);

    Sleep(1000);

    EnumWindows(EnumWindowProc, (LPARAM)SW_SHOW);
}
else {
    std::cout << GetLastError() << std::endl;
}

EnumWindowProc is just for showing and hiding system background images.