display flex overflow hidden slider

.slider {
  display: flex;
  width: 300px;      /* just for this demo */
  margin: 0 auto;    /* just for this demo */
  background: gold;  /* just for this demo */
 overflow: hidden; /* Uncomment once you understand the demo */
}

.item {
  flex: 1 0 50%;
  outline: 1px solid red;
  height: 150px;
}