Force Last Div on a New Line with Flexbox

Move last element to next line in a flex container

Apply flex-wrap to the container.

Apply flex: 1 0 33% to the first three child divs.

To the fourth div apply flex-basis: 100%; display: flex.

To the children of the fourth div apply flex: 1 0 33%.

The idea is to force the fourth item to wrap, then have its children replicate the behavior of the children in the primary container.

Flexbox item wrap to a new line

If you look at this great answer you'll notice that the only cross-browser way (without 2 line break limit) is inserting 100%-width empty blocks ("line-breaks"). So for similar markup this will look like

.flex {  display: flex;  flex-wrap: wrap;  border: 2px solid red;}
.item {  width: 50px;  height: 50px;  margin: 5px;  border: 2px solid blue;}
.line-break {  width: 100%;}

<div class="flex">  <div class="item"></div>  <div class="item"></div>  <div class="item"></div>  <div class="line-break"></div>  <div class="item"></div>  <div class="item"></div>  <div class="line-break"></div>  <div class="item"></div>  <div class="line-break"></div>  <div class="item"></div>  <div class="item"></div>  <div class="item"></div></div>

How to specify line breaks in a multi-line flexbox layout?

The simplest and most reliable solution is inserting flex items at the right places. If they are wide enough (width: 100%), they will force a line break.

.container {
  background: tomato;
  display: flex;
  flex-flow: row wrap;
  align-content: space-between;
  justify-content: space-between;
}
.item {
  width: 100px;
  background: gold;
  height: 100px;
  border: 1px solid black;
  font-size: 30px;
  line-height: 100px;
  text-align: center;
  margin: 10px
}
.item:nth-child(4n - 1) {
  background: silver;
}
.line-break {
  width: 100%;
}

<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="line-break"></div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
  <div class="line-break"></div>
  <div class="item">7</div>
  <div class="item">8</div>
  <div class="item">9</div>
  <div class="line-break"></div>
  <div class="item">10</div>
</div>

Targeting flex items on the last or specific row

Unfortunately, in the current iteration of flexbox (Level 1), there is no clean way to solve the last-row alignment problem. It's a common problem.

It would be useful to have a flex property along the lines of:

last-row
last-column
only-child-in-a-row
alone-in-a-column

This problem does appear to be a high priority for Flexbox Level 2:

CSS Working Group Wiki - Specification Issues and Planning
https://lists.w3.org/Archives/Public/www-style/2015Jan/0150.html

Although this behavior is difficult to achieve in flexbox, it's simple and easy in CSS Grid Layout:

Equal width flex items even after they wrap

In case Grid is not an option, here's a list of similar questions containing various flexbox hacks:

Properly sizing and aligning the flex item(s) on the last row
Flex-box: Align last row to grid
Flexbox wrap - different alignment for last row
How can a flex item keep the same dimensions when it is forced to a new row?
Selector for an element alone in a row?
Aligning elements in last flexbox row
How can I allow flex-items to grow while keeping the same size?
Left-align last row of flexbox using space-between and margins
Inconsistent margin between flex items on last row
How to keep wrapped flex-items the same width as the elements on the previous row?
How to align left last row/line in multiple line flexbox
Last children of grid get giant gutter cause of flexbox space-between
Managing justify-content: space-between on last row
Flexbox space between behavior combined with wrap
Possible to use CSS Flexbox to stretch elements on every row while maintaining consistent widths?

Force flex item to span full row width

When you want a flex item to occupy an entire row, set it to width: 100% or flex-basis: 100%, and enable wrap on the container.

The item now consumes all available space. Siblings are forced on to other rows.

.parent {
  display: flex;
  flex-wrap: wrap;
}

#range, #text {
  flex: 1;
}

.error {
  flex: 0 0 100%; /* flex-grow, flex-shrink, flex-basis */
  border: 1px dashed black;
}

<div class="parent">
  <input type="range" id="range">
  <input type="text" id="text">
  <label class="error">Error message (takes full width)</label>
</div>