To make your application process input messages resulting from double-clicking the left mouse button, add a WM_LBUTTONDBLCLK case to the window procedure, as follows:
case WM_LBUTTONDBLCLK:
wsprintf(ButtonText, "WM_LBUTTONDBLCLK: %x, %d, %d",
wParam, LOWORD(lParam), HIWORD(lParam));
InvalidateRect(hWnd, &rectButton, TRUE);
break;