How Does Margin Affect Flex-Grow Sizing in Flexbox Layouts?

Sizing Flex Elements Using Flex-Grow

In flexbox layouts, flex items are sized according to their flex-grow property, which specifies how much free space each item should occupy. However, there are certain factors that can affect the calculated size of flex items, including content and margins.

Issue Overview

Consider the following code:

.numbers {
    display: flex;
    flex-direction: column;
}

.row {
    display: flex;
    flex-direction: row;
    flex-grow: 1;
    justify-content: space-between;
}

.button {
    display: flex;
    flex-grow: 1;
    justify-content: center;
    align-items: center;
    margin: 5px;
    border-radius: 5px;
    border: 1px solid gray;
    background: rgba(255, 255, 255, 0.2);
    cursor: pointer;
}

#number0 {
    flex-grow: 2;
}

#colon {
    flex-grow: 1;
}

<div class="numbers">
    <div class="row">
        <div class="button number">

In this example, we expect the "0" button to be twice as wide as the other buttons, and the colon button to be half as wide. However, the alignment of these buttons appears to be slightly off.

Cause of the Issue

The issue arises because the space available on each line is determined by the total width of the buttons and the margins applied to them. In our example, the first three rows have two horizontal margins each, while the fourth row has only one. This results in the fourth row having more free space than the other rows, which affects the distribution of space among the flex items.

Using Flex-Basis for More Reliable Sizing

To address this issue, it's recommended to use flex-basis instead of flex-grow for more reliable sizing of flex items. Flex-basis sets the initial size of the item, taking into account its content and margins. By specifying the flex-basis for each button, we can ensure that they are all given an equal amount of space, regardless of the margins.

Here's the corrected code:

.button {
    flex-basis: 33.33%;
}

#number0 {
    flex-basis: calc(66.67% + 10px);
}

* {
    box-sizing: border-box;
}

By updating the flex-basis, we can achieve the desired alignment of the buttons, with the "0" button being twice as wide as the other buttons and the colon button being half as wide.

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