Answers for "flex stretch to fill width"

flexbox stretch height

.flex-2 {
    display: flex;  
    align-items: stretch;
}

flex one whole width

.flexContainer {
  height: 100vh;
  display: flex;
  flex-wrap: wrap;
}

.item:first-child {
  flex: 1 1 100%;
  margin-bottom: 20px;
}

.item {
  flex: 1 1 40%;
  height: 50px;
  background-color: red;
  margin: 0 20px;
}

flex fill space

flex-grow: 1