When Does Flex-Grow Switch to Flex-Shrink, and Vice-Versa

When does flex-grow switch to flex-shrink, and vice-versa?

The flex-basis property sets the initial main size of the flex item, before free space is distributed by other flex properties.

This means that the flex item is first sized for its defined width (in flex-direction: row) or height (in flex-direction: column).

In other words, in a row-direction flex container, where flex-basis: 100px is equivalent to width: 100px, the item would have a width of 100px.

That's the initial main size.

THEN, other flex properties are applied. Namely, flex-grow and flex-shrink.

If the container is wider than 100px, then flex-grow: 1 expands the item to fill the extra space.

If the container is less than 100px, then flex-grow: 1 is ignored and flex-shrink: 1 reduces the size of the item according to a flex sizing algorithm.

So, to answer your question:

When does flex switch from grow to shrink?

Assuming both properties are enabled (i.e., they are not flex-grow: 0 or flex-shrink: 0), the switch will occur when the sum total of flex-basis / width / height on flex items is no longer less or more than the length of the container.

To make this a bit more clear:

When the sum total of flex-basis / width / height on flex items is no longer more than the length of the container, this means there is no overflow and the items don't consume all available space. flex-grow works.
When the sum total of flex-basis / width / height on flex items is no longer less than the length of the container, this means there is overflow and the items must shrink to fit inside the container. flex-shrink works.

From the spec:

flex-grow

This [property] specifies the flex grow factor, which determines how much the flex item will grow relative to the rest of
the flex items in the flex container when positive free space is
distributed.

flex-shrink

This [property] specifies the flex shrink factor, which determines how much the flex item will shrink relative to the rest of the flex
items in the flex container when negative free space is distributed.

flex-basis

This [property] specifies the initial main size of the flex item,
before free space is distributed according to the flex factors.

_{https://www.w3.org/TR/css-flexbox-1/#flex-property}

Understanding flex-grow and flex-shrink when using flex-basis

Percentage lengths are relative to their containing blocks.

Therefore, if the flex container has a width of 200px, and the flex items are set to flex-basis: 50%, then each item will resolve to 100px.

Of course, in flex layout, flex-basis represents the initial main size or, the size before flex-grow and flex-shrink are applied.

You have flex-grow disabled, so nothing happens there.

But you have flex-shrink enabled, so the items will shrink below 100px when necessary to prevent an overflow of the container.

In this case, because all items are set to flex-shrink: 1, they will shrink in equal proportion.

article {

  display: flex;

  width: 200px;

  border: 1px solid black;

}

[one] > section {

  flex: 0 1 50px;

}

[two] > section {

  flex: 0 1 50%;

}

[three] > section {

  flex: 0 0 50%;

}

/* non-essential demo styles */

section {

  height: 50px;

  background-color: lightgreen;

  border: 1px solid #ccc;

  display: flex;

  justify-content: center;

  align-items: center;

  box-sizing: border-box;

}

<p>container width 200px in all cases</p>

<article one>

  <section><span>50px</span></section>

  <section><span>50px</span></section>

  <section><span>50px</span></section>

  <section><span>50px</span></section>

</article>

<hr>

<p><code>flex-shrink</code> enabled</p>

<article two>

  <section><span>50%</span></section>

  <section><span>50%</span></section>

  <section><span>50%</span></section>

  <section><span>50%</span></section>

</article>

<hr>

<p><code>flex-shrink</code> disabled</p>

<article three>

  <section><span>50%</span></section>

  <section><span>50%</span></section>

  <section><span>50%</span></section>

  <section><span>50%</span></section>

</article>

In what circumstances is flex-shrink applied to flex elements and how does it work?

In order to see flex-shrink in action, you need to be able to make its container smaller.

HTML:

<div class="container">
  <div class="child one">
    Child One
  </div>

  <div class="child two">
    Child Two
  </div>
</div>

CSS:

div {
  border: 1px solid;
}

.container {
  display: flex;
}

.child.one {
  flex: 1 1 10em;
  color: green;
}

.child.two {
  flex: 2 2 10em;
  color: purple;
}

http://jsfiddle.net/GyXxT/ (unprefixed -- Opera or Firefox nightly build)
http://jsfiddle.net/GyXxT/1/ (webkit)

In this example, both child elements ideally want to be 10em wide. If the parent element is greater than 20em wide, the 2nd child will take twice as much leftover space as the 1st child, making it appear bigger. If the parent element is less than 20em wide, then the 2nd child will have twice as much shaved off of it as the 1st child, making it look smaller.

Current flexbox support: Opera (unprefixed), Chrome (prefixed), IE10 (prefixed, but uses slightly different property names/values). Firefox currently uses the old spec from 2009 (prefixed), but the new spec is supposed to be available in experimental builds right now (unprefixed).

What are the differences between flex-basis and width?

Consider `flex-direction`

The first thing that comes to mind when reading your question is that flex-basis doesn't always apply to width.

When flex-direction is row, flex-basis controls width.

But when flex-direction is column, flex-basis controls height.

Key Differences

Here are some important differences between flex-basis and width / height:

flex-basis applies only to flex items. Flex containers (that aren't also flex items) will ignore flex-basis but can use width and height.
flex-basis works only on the main axis. For example, if you're in flex-direction: column, the width property would be needed for sizing flex items horizontally.
flex-basis has no effect on absolutely-positioned flex items. width and height properties would be necessary. Absolutely-positioned flex items do not participate in flex layout.
By using the flex property, three properties – flex-grow, flex-shrink and flex-basis – can be neatly combined into one declaration. Using width, the same rule would require multiple lines of code.

Browser Behavior

In terms of how they are rendered, there should be no difference between flex-basis and width, unless flex-basis is auto or content.

From the spec:

7.2.3. The flex-basis property

For all values other than auto and content, flex-basis is resolved the same way as width in horizontal writing modes.

But the impact of auto or content may be minimal or nothing at all. More from the spec:

auto

When specified on a flex item, the auto keyword retrieves the value
of the main size property as the used flex-basis. If that value is
itself auto, then the used value is content.

content

Indicates automatic sizing, based on the flex item’s content.

Note: This value was not present in the initial release of Flexible
Box Layout, and thus some older implementations will not support it.
The equivalent effect can be achieved by using auto together with a
main size (width or height) of auto.

So, according to the spec, flex-basis and width resolve identically, unless flex-basis is auto or content. In such cases, flex-basis may use content width (which, presumably, the width property would use, as well).

The `flex-shrink` factor

It's important to remember the initial settings of a flex container. Some of these settings include:

flex-direction: row - flex items will align horizontally
justify-content: flex-start - flex items will stack at the start of the line on the main axis
align-items: stretch - flex items will expand to cover the cross-size of the container
flex-wrap: nowrap - flex items are forced to stay in a single line
flex-shrink: 1 - a flex item is allowed to shrink

Note the last setting.

Because flex items are allowed to shrink by default (which prevents them from overflowing the container), the specified flex-basis / width / height may be overridden.

For example, flex-basis: 100px or width: 100px, coupled with flex-shrink: 1, will not necessarily be 100px.

To render the specified width – and keep it fixed – you will need to disable shrinking:

div {
   width: 100px;
   flex-shrink: 0;
}

div {
  flex-basis: 100px;
  flex-shrink: 0;
}

OR, as recommended by the spec:

flex: 0 0 100px;    /* don't grow, don't shrink, stay fixed at 100px */

7.2. Components of
Flexibility

Authors are encouraged to control flexibility using the flex shorthand
rather than with its longhand properties directly, as the shorthand
correctly resets any unspecified components to accommodate common
uses.

Browser Bugs

Some browsers have trouble sizing flex items in nested flex containers.

`flex-basis` ignored in a nested flex container. `width` works.

When using flex-basis, the container ignores the sizing of its children, and the children overflow the container. But with the width property, the container respects the sizing of its children and expands accordingly.

References:

Chrome does not expand flex parent according to children's content
Flex item overflowing when using flex-basis
Difference between width and flex-basis
Flex-basis is being ignored when sizing nested flex containers.
flex-basis:100px does something different from width:100px+flex-basis:auto

Examples:

https://jsfiddle.net/t419zhra/ (source: @Dremora)
https://codepen.io/anon/pen/NVxaoy (source @Daniel)
https://jsfiddle.net/voc9grx6/ (source: Chromium Bugs)
https://jsfiddle.net/qjpat9zk/ (source: Chromium Bugs)

flex items using `flex-basis` and `white-space: nowrap` overflow `inline-flex` container. `width` works.

It seems that a flex container set to inline-flex doesn't recognize flex-basis on a child when rendering a sibling with white-space: nowrap (although it could just be an item with undefined width). The container doesn't expand to accommodate the items.

But when the width property is used instead of flex-basis, the container respects the sizing of its children and expands accordingly. This is not a problem in IE11 and Edge.

References:

inline flex container width not growing
Inline flex container (display: inline-flex) is expanding the full width of parent container

Example:

https://jsfiddle.net/p18h0jxt/1/ (from first post above)

`flex-basis` (and `flex-grow`) not working on table element

References:

Why does flex-box work with a div, but not a table?
Why doesn't flex-grow: 1 work for a table in Safari? (and Edge)

`flex-basis` fails in Chrome and Firefox when the grandparent container is a shrink-to-fit element. The set-up works fine in Edge.

Absolutely positioned container not expanding width to fit flexbox content

Like in the example presented in the link above, involving position: absolute, the use of float and inline-block, will also render the same flawed output (jsfiddle demo).

Bugs affecting IE 10 and 11:

flex shorthand declarations with unitless flex-basis values are ignored
flex-basis doesn't account for box-sizing: border-box
flex-basis doesn't support calc()
Importance is ignored on flex-basis when using flex shorthand

How does flex-shrink factor in padding and border-box?

Flexbox defines this as

For every unfrozen item on the line, multiply its flex shrink factor
by its inner flex base size, and note this as its scaled flex
shrink factor. Find the ratio of the item’s scaled flex shrink
factor to the sum of the scaled flex shrink factors of all
unfrozen items on the line. Set the item’s target main size to
its flex base size minus a fraction of the absolute value of the
remaining free space proportional to the ratio.

Simplifying, frozen flex items are those which can't or don't have to be flexed anymore. I will assume no min-width restrictions and non-zero flex shrink factors. This way all flex items are initially unfrozen and they all become frozen after only one iteration of the flex loop.

The inner flex base size depends on the value of box-sizing, defined by CSS2UI as

content-box: The specified width and height (and respective min/max properties) apply to the width and height respectively of the
content box of the element. The padding and border of the element are
laid out and drawn outside the specified width and height.

border-box: Length and percentages values for width and height (and respective min/max properties) on this element determine the
border box of the element. That is, any padding or border specified on
the element is laid out and drawn inside this specified width
and height. The content width and height are calculated by
subtracting the border and padding widths of the respective sides from
the specified width and height properties. [...] Used values, as
exposed for instance through getComputedStyle(), also refer to the
border box.

Basically, that means that sizes (widths, flex bases) have an inner an an outer variant. The inner size includes only the content, the outer one also includes paddings and border widths. The length specified in the stylesheet will be used as the inner size in case of box-sizing: content-box, or as the outer one in case of box-sizing: border-box. The other can be calculated by adding or subtracting border and padding widths.

Neglecting lots of details, the algorithm would be something like

let sumScaledShrinkFactors = 0,
    remainingFreeSpace = flexContainer.innerMainSize;
for (let item of flexItems) {
  remainingFreeSpace -= item.outerFlexBasis;
  item.scaledShrinkFactor = item.innerFlexBasis * item.flexShrinkFactor;
  sumScaledShrinkFactors += item.scaledShrinkFactor;
}
for (let item of flexItems) {
  let ratio = item.scaledShrinkFactor / sumScaledShrinkFactors;
  item.innerWidth = item.innerFlexBasis + ratio * remainingFreeSpace;
}

With no paddings, it's as you explain

                                   (width)
                                   innerW │ padd │ outerW
                                   ───────┼──────┼───────
300px * (1 + 2 / 1000px * -200px) = 180px │  0px │  180px
200px * (1 + 1 / 1000px * -200px) = 160px │  0px │  160px
100px * (1 + 2 / 1000px * -200px) =  60px │  0px │   60px
                                   ───────┼──────┼───────
                                    400px │  0px │  400px

With 10px horizontal paddings, the available space is reduced by 3 * 2 * 10px = 60px, so now it's -260px.

                                   (width)
                                   innerW │ padd │ outerW
                                   ───────┼──────┼───────
300px * (1 + 2 / 1000px * -260px) = 144px │ 20px │  164px
200px * (1 + 1 / 1000px * -260px) = 148px │ 20px │  168px
100px * (1 + 2 / 1000px * -260px) =  48px │ 20px │   68px
                                   ───────┼──────┼───────
                                    340px │ 60px │  400px

When you use box-sizing: border-box, the specified flex bases are the outer ones, so the paddings are subtracted from them to calculate the inner ones, which are 280px, 180px, 80px. Then the sum of the scaled flex shrink factors becomes 2*280px + 180px + 2*80px = 900px. The available space is like in the case with no padding, because the outer flex bases are the same. Note the width retrieved by getComputedStyle will now be the outer one, so paddings are added back at the end.

                                                    (width)
                                    innerW │ padd │  outerW
                                   ────────┼──────┼────────
280px * (1 + 2 / 900px * -200px) ≈ 155.6px │ 20px │ 175.6px
180px * (1 + 1 / 900px * -200px) = 140.0px │ 20px │ 160.0px
 80px * (1 + 2 / 900px * -200px) ≈  44.4px │ 20px │  64.4px
                                   ────────┼──────┼────────
                                   340.0px │ 60px │ 400.0px

What does flex: 1 mean?

Here is the explanation:

https://www.w3.org/TR/css-flexbox-1/#flex-common

flex: <positive-number>
Equivalent to flex: <positive-number> 1 0. Makes the flex item flexible and sets the flex basis to zero, resulting in an item that
receives the specified proportion of the free space in the flex
container. If all items in the flex container use this pattern, their
sizes will be proportional to the specified flex factor.

Therefore flex:1 is equivalent to flex: 1 1 0

When Does Flex-Grow Switch to Flex-Shrink, and Vice-Versa