Elliptic Window Flickering Problem

#include <windows.h>
#include <windowsx.h>

HWND CreateTestWindow(WNDPROC);

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) {
    static bool moving{};

    switch (msg) {
        case WM_DESTROY: {
            PostQuitMessage(0);
            return 0;
        }

        case WM_LBUTTONDOWN: {
            moving = true;
            return 0;
        }

        case WM_LBUTTONUP: {
            moving = false;
            return 0;
        }

        case WM_MOUSEMOVE: {
            static POINT old{0, 0};

            if (moving) {
                RECT r;
                GetWindowRect(hwnd, &r);

                int x = GET_X_LPARAM(lParam);
                int y = GET_Y_LPARAM(lParam);

                RECT newPos;
                newPos.left = r.left + x - old.x;
                newPos.top = r.top + y - old.y;

                SetWindowRgn(hwnd, nullptr, FALSE);

                SetWindowPos(hwnd, nullptr, newPos.left, newPos.top, 0, 0,
                    SWP_NOZORDER | SWP_NOSIZE | SWP_NOREDRAW
                );

                SetWindowRgn(hwnd, CreateEllipticRgn(200, 200, 600, 400), FALSE);
            }

            old.x = GET_X_LPARAM(lParam);
            old.y = GET_Y_LPARAM(lParam);

            return 0;
        }
    }

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

int main() {
    HWND hwnd = CreateTestWindow(WndProc);
    SetWindowRgn(hwnd, CreateEllipticRgn(200, 200, 600, 400), TRUE);

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

HWND CreateTestWindow(WNDPROC f) {
    WNDCLASS wc;
    ZeroMemory(&wc, sizeof wc);
    wc.hInstance = GetModuleHandle(nullptr);
    wc.lpfnWndProc = f;
    wc.lpszClassName = "Test Window Class";
    wc.hbrBackground = reinterpret_cast<HBRUSH>(COLOR_WINDOW + 1);

    RegisterClass(&wc);

    return CreateWindow("Test Window Class", "Test Window",
        WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        nullptr, nullptr, GetModuleHandle(nullptr), nullptr
    );
}