Flex items overflowing container in Safari

For some reason, flex items are not staying inside the container in Safari.

Here's what the layout looks like in Chrome and Firefox:

Here's what it looks like in Safari:

Here's my code:

#div1 {
  background: black;
  width: 250px;
  height: 600px;
  display: flex;
  flex-direction: column;
  padding-bottom: 5px
}

#div2 {
  background: pink;
  color: #FFF;
  width: 240px;
  height: 200px
}

#div3 {
  background: blue;
  color: #FFF;
  width: 240px;
  height: 100%;
  position: relative
}

<div id="div1">
  <div id="div2">test</div>
  <div id="div3">test2</div>
</div>

The Problem

You have a container with height: 600px.

This container has two children:

One child has height: 200px.
The other child has height: 100%.

Since percentage heights are based on the height of the parent, you are setting the height of the second child to equal the full height of the container.

10.5 Content height: the height property

percentage

Specifies a percentage height. The percentage is calculated with respect to the height of the generated box's containing block.

As a result, an overflow occurs:

(200px + 600px) > 600px

Except that in a flex container, an initial setting is flex-shrink: 1. This means that flex items can shrink in order to fit inside the container. Chrome and Firefox apply this setting correctly, allowing the element with height: 100% to shrink to fit. Safari, apparently, has a different interpretation.

Solutions

You could use calc() to solve the problem:

#div3 {
  height: calc(100% - 200px);
}

#div1 {
  background: black;
  width: 250px;
  height: 600px;
  display: flex;
  flex-direction: column;
  padding-bottom: 5px
}

#div2 {
  background: pink;
  color: #FFF;
  width: 240px;
  height: 200px
}

#div3 {
  background: blue;
  color: #FFF;
  width: 240px;
  height: calc(100% - 200px);
}

<div id="div1">
  <div id="div2">test</div>
  <div id="div3">test2</div>
</div>

However, since you're already working in a column-direction flex container, you can use flex to make the second child consume remaining space:

#div3 {
  flex: 1;
}

This means: Any space not used up by other siblings will be consumed by this child.

#div1 {
  background: black;
  width: 250px;
  height: 600px;
  display: flex;
  flex-direction: column;
  padding-bottom: 5px
}

#div2 {
  background: pink;
  color: #FFF;
  width: 240px;
  height: 200px
}

#div3 {
  background: blue;
  color: #FFF;
  width: 240px;
  flex: 1;
}

<div id="div1">
  <div id="div2">test</div>
  <div id="div3">test2</div>
</div>