在 Flex 容器中没有收缩的 input/Button 元素

An input element, unlike a div, comes with a default width.

Here's a simple illustration of this setting:

The browser automatically gives the input a width.

input {
border: 1px solid blue;
display: inline;
}


div {
border: 1px solid red;
display: inline;
}

<form>
<input>
<br><br>
<div></div>
</form>

Also, an initial setting on flex items is min-width: auto. This means that items cannot shrink below their width on the main axis.

Hence, input elements cannot shrink below their default width and may be forced to overflow the flex container.

You can override this behavior by setting your inputs to min-width: 0 (revised codepen)

Here's a full explanation: Why don't flex items shrink past content size?

In some cases, you may need to override input widths using width: 100% or width: 0.

I would like to extend @Michael Benjamin solution

In some cases, you may need to override input widths using width: 100% or width: 0.

you can also do calc(100%)

if the parent is a flexible container set with the usual flex: 1 (flex-grow: 1) and you want the input to shrink with the parent, this is what has worked for me:

width: 35px;
flex: 1 2 0px;

Give this as a class to the input, this will make it take the whole space when its available, but shrink to minimum size when resized.

Edit:

This works because it's a shorthand for flex: <flex-grow> <flex-shrink> <flex-basis> as explained here https://css-tricks.com/snippets/css/a-guide-to-flexbox/#flex

The '2' is assigned to flex-shrink which makes the element shrink. Sometimes you still need to add min-width: 0; as mentioned in other comments to remove the min-width defined by browsers (for small containers)

Use flex-basis: 0; for input or input container solves the problem.

I needed to wrap the input element with a styled element and than set the input width and min-width as below:

.field__input input {
width: 0;
min-width: 100%;
}

If you have an input element inside a flex box, you can define width: 0 and still can utilize flex: 1 or whatever value you need.

This is Christof Kälin's answer in the comments for another answer. Putting it here for visibility.

const
setValue = (btn, amount) => {
const input = btn.closest('.stepper').querySelector('.stepper-val');
input.value = +input.value + amount;
},
handleDecr = e => setValue(e.target, -1),
handleIncr = e => setValue(e.target, 1);


document.querySelectorAll('.stepper').forEach(s => {
s.querySelector('.stepper-minus').addEventListener('click', handleDecr);
s.querySelector('.stepper-plus').addEventListener('click', handleIncr);
});

html {
font-family: Arial, Helvetica, sans-serif;
/* font-size: 3vw; */
}


html, body, div, button, input {
box-sizing: border-box;
}


#atf {
height: 100vh;
/* flex-container */
display: flex;
justify-content: center;
align-items: center;
}


.stepper {
/* flex-container */
display: flex;
/* formatting */
width: 12rem;
height: 2rem;
}


.stepper-minus {
/* flex-item */
flex-grow: 1;
/* flex-container */
display: flex;
justify-content: center;
align-items: center;
/* formatting */
background-color: deepskyblue;
border: 1px solid lightgray;
border-radius: .6rem 0 0 .6rem;
cursor: pointer;
}


.stepper-val {
/* flex-item */
flex-grow: 2;
/* flex-container */
text-align: center;
/* formatting */
border-top: 1px solid lightgray;
border-bottom: 1px solid lightgray;
border-left: none;
border-right: none;
width: 0;
}


.stepper-plus {
/* flex-item */
flex-grow: 1;
/* flex-container */
display: flex;
justify-content: center;
align-items: center;
/* formatting */
background-color: deepskyblue;
border: 1px solid lightgray;
border-radius: 0 .6rem .6rem 0;
cursor: pointer;
}


.stepper>button:hover {
background-color: blue;
}

<div id="atf">
<div class="stepper">
<button class="stepper-minus">&minus;</button>
<input class="stepper-val" type="text" name="val" value="32">
<button class="stepper-plus">&plus;</button>
</div>
</div>

Edit: Making this answer a community wiki so that I don't get unearned credit.

I'd recommend setting the width to 100% or what ever percentage you want but you should set it. That works for me.