setting initial width of items in flexbox

Question

I'm having a problem with flexbox, I'm trying to get my first .cell1 to take on the width of the 3 items inside it so it should be 300px wide so I have set this to flex: 0 0 auto and I have set .cell2 to flex: 1 so it takes up the remaining space but instead of doing this it seems to overlap .cell1 and I'm not sure why?

How can I get .cell1 to take on the width of the 3 items inside it and .cell2 to take the remaining width?

.wrap {
  display:flex;
  flex-wrap:wrap;
  width: 100%;
}

.cell1{
  display:flex;
  flex: 0 0 auto;
  flex-direction: row;
}
.cell2{
  display: flex;
  flex: 1;
  background: #ff0000;
}
.item1{
  flex: 0 0 100px;
  background: #ff0000;
}

.item2{
  flex: 0 0 100px;
  background: #0000ff;
}

.item3{
  flex: 0 0 100px;
  background: #ff00ff;
}

<div class="wrap">
  <div class="cell1">
    <div class="item1">
    item 1
    </div>
    <div class="item2">
    item 2
    </div>
    <div class="item3">
    item 3
    </div>
  </div>

  <div class="cell2">
  cell2
  </div>
</div>

Answer 1

Use width instead of flex-basis. This is a known bug that you can read more about here :

What are the differences between flex-basis and width? (At the end of the accepted answer by @Michael_B)

Bug affecting all major browsers, except IE 11 & Edge:

flex-basis ignored in a nested flex container. width works. A problem has been found when sizing flex items in a nested flex container.

.wrap {
  display:flex;
  flex-wrap:wrap;
  width: 100%;
}

.cell1{
  display:flex;
  flex-direction: row;
}
.cell2{
  display: flex;
  flex: 1;
  background: #ff0000;
}
.item1{
  width:100px;
  background: #ff0000;
}

.item2{
  width:100px;
  background: #0000ff;
}

.item3{
  width:100px;
  background: #ff00ff;
}

<div class="wrap">
  <div class="cell1">
    <div class="item1">
    item 1
    </div>
    <div class="item2">
    item 2
    </div>
    <div class="item3">
    item 3
    </div>
  </div>

  <div class="cell2">
  cell2
  </div>
</div>