A bola sempre será reconstruída, enquanto o programa receber uma mensagem WM_SIZE - Post reescrito em 30/09/2023

#include <windows.h>
#include "resource.h"

#define ID_TIMER    1

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

int WINAPI WinMain(HINSTANCE hI, HINSTANCE hPI, PSTR szCL, int iCS) {
    static TCHAR nome[] = TEXT("Bounce");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;

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

    if(!RegisterClass(&wndclass)) {
        MessageBox(NULL, TEXT("Error!"), nome, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(
        nome, "Programação para Windows", WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, 400, 400,
        NULL, NULL, hI, NULL
    );

    ShowWindow(hwnd, iCS);

    UpdateWindow(hwnd);

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

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wP, LPARAM lP) {
    static HBITMAP hBmp;
    static int cxClient, cyClient, xCtr, yCtr, cxTotal, cyTotal,
        cxRadius, cyRadius, cxMove, cyMove, xPixel, yPixel;
    HBRUSH hBrush;
    HDC    hdc, hdc2;
    int    iScale;
    //xCtr = xCenter, yCtr = yCenter
    switch(msg) {
        case WM_CREATE:
            hdc = GetDC(hwnd);
            xPixel = GetDeviceCaps(hdc, ASPECTX);
            yPixel = GetDeviceCaps(hdc, ASPECTY);
            ReleaseDC(hwnd, hdc);
            SetTimer(hwnd, ID_TIMER, 20, NULL);
            return 0;

        case WM_SIZE:
            xCtr = (cxClient = LOWORD(lP)) / 2;
            yCtr = (cyClient = HIWORD(lP)) / 2;

            iScale = min(
                cxClient * xPixel, cyClient * yPixel
            ) / 16;

            cxRadius = iScale / xPixel;
            cyRadius = iScale / yPixel;
            cxMove = max(1, cxRadius / 2);
            cyMove = max(1, cyRadius / 2);
            cxTotal = 2 * (cxRadius + cxMove);
            cyTotal = 2 * (cyRadius + cyMove);

            if(hBmp) {
                DeleteObject(hBmp);
            }

            hdc = GetDC(hwnd);
            hdc2 = CreateCompatibleDC(hdc);

            hBmp = CreateCompatibleBitmap(
                hdc, cxTotal, cyTotal
            );

            ReleaseDC(hwnd, hdc);

            SelectObject(hdc2, hBmp);

            Rectangle(hdc2, -1, -1, cxTotal + 1, cyTotal + 1);

            hBrush = CreateHatchBrush(HS_DIAGCROSS, 0L);
            SelectObject(hdc2, hBrush);
            SetBkColor(hdc2, RGB(0, 243, 60));

            Ellipse(
                hdc2, cxMove, cyMove,
                cxTotal - cxMove, cyTotal - cyMove
            );

            DeleteDC(hdc2);
            DeleteObject(hBrush);
            return 0;
        case WM_TIMER:
            if(!hBmp) {
                break;
            }
            hdc = GetDC(hwnd);
            hdc2 = CreateCompatibleDC(hdc);
            SelectObject(hdc2, hBmp);

            BitBlt(
                hdc, xCtr - cxTotal / 2,
                yCtr - cyTotal / 2, cxTotal,
                cyTotal, hdc2, 0, 0, SRCCOPY
            );

            ReleaseDC(hwnd, hdc);
            DeleteDC(hdc2);
            xCtr += cxMove;
            yCtr += cyMove;

            if((xCtr+cxRadius>=cxClient)||(xCtr-cxRadius<=0)) {
                cxMove = -cxMove;
            }

            if((yCtr+cyRadius>=cyClient)||(yCtr-cyRadius<=0)) {
                cyMove = -cyMove;
            }
            return 0;
        case WM_DESTROY:
            if(hBmp) {
                DeleteObject(hBmp);
            }

            KillTimer(hwnd, ID_TIMER);
            PostQuitMessage(0);
            return 0;
    }
    return DefWindowProc(hwnd, msg, wP, lP);
}

//Arquivo: resource.h

#define ICONE 100

//Arquivo: resource.rc

#include "resource.h"
#include "afxres.h"


ICONE ICON "icon3.ico"