margin with flex wrap

Question

I am trying to add some spacing between the divs using display: flex and flex-wrap: wrap.

The problem is that when I apply margin-right to the second item, the row breaks. How can I add some spacing between the items without breaking them in 2 rows?

jsFiddle

* {
  box-sizing: border-box;
}
// Default
// ----------------------------------------------------
.collage {
  position: relative;
  display: flex;
  justify-content: flex-start;
  align-items: center;
  flex-wrap: wrap;
  margin-bottom: 32px;
}
.collage-item {
  width: 100%;
  height: 25vw;
  background: url("https://www.audi.co.uk/content/dam/audi/production/Models/NewModelsgallery/A5range/A5_Coupe/MY17/1920x1080_A5-Coupe-2016-side.jpg") no-repeat;
  background-size: cover;
  flex: 0 0 50%;
  &: nth-child(1) {
    margin-bottom: 16px;
  }
  &:nth-child(2) {
    margin-right: 16px;
  }
  &:nth-child(4) {
    margin-top: 16px;
  }
  &:nth-child(1),
  &:nth-child(4) {
    flex: 0 0 100%;
    height: 50vw;
  }
}
// Button
// ----------------------------------------------------
 .btn {
  position: absolute;
  border: 2px solid white;
  padding: 10px 18px;
  text-align: center;
  right: 16px;
  bottom: 16px;
  color: white;
  text-decoration: none;
}

<div class="collage">
  <div class="collage-item"></div>
  <div class="collage-item"></div>
  <div class="collage-item"></div>
  <div class="collage-item"></div>
  <div class="btn">View all 11 photos</div>
</div>

Answer 1

You are setting the basis to 50% then when you add the margin it push the next element since can't fit side by side anymore. You may want to let the items grow and avoid the basis:

.collage-item {
  flex: 1 0 auto;
}

Jsfiddle Demo

Answer 2

You have both second row images set to 50% width:

flex: 0 0 50%;

When you add the 16px horizontal margin, you exceed the total width of the row, forcing a wrap:

50% + 50% + 16px = overflow

Try factoring the margin space into the width of the images:

flex: 0 0 calc(50% - 8px);

revised fiddle