How to start a new column in flex column wrap layout

I want my data to be arranged in columns (top to bottom, left to right) and every heading inside the data should start a new column. There are three constraints:

Must use flex (I need to use a feature specific to flex)
Cannot group the data (e.g. wrap all data items inside one div)
Cannot set fixed height

My question is how do I force a column break inside a flex-flow: column wrap layout?

.grid {
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
}
.grid .head {
  width: 25%;
  background: orange;
  border-bottom: thin dotted;
}
.grid .data {
  width: 25%;
  background: yellow;
  border-bottom: thin dotted;
}
/* my attempt to solve this */
.grid {
  height: 76px;
}

<div class="grid">
  <div class="head">Column 1 (3 items)</div>
  <div class="data">item 1-1</div>
  <div class="data">item 1-2</div>
  <div class="data">item 1-3</div>
  <div class="head">Column 2 (4 items)</div>
  <div class="data">item 2-1</div>
  <div class="data">item 2-2</div>
  <div class="data">item 2-3</div>
  <div class="data">item 2-4</div>
  <div class="head">Column 3 (2 items)</div>
  <div class="data">item 3-1</div>
  <div class="data">item 3-2</div>
  <div class="head">Column 4 (1 items)</div>
  <div class="data">item 4-1</div>
</div>

Solution

Apparently, the correct solution is to use the break-before or break-after property:

A break is forced wherever the CSS2.1 page-break-before/page-break-after [CSS21] or the CSS3 break-before/break-after [CSS3-BREAK] properties specify a fragmentation break.

At the time of writing, most browsers implement the *-break-* properties incorrectly or do not implement them at all. Consider this answer ahead of its time.

The following demo works in:

FF33

.grid {
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
}
.grid .head {
  width: 25%;
  background: orange;
  border-bottom: thin dotted;
}
.grid .data {
  width: 25%;
  background: yellow;
  border-bottom: thin dotted;
}
/* force column breaks */
.grid .head:nth-child(n + 2) {
  page-break-before: always; /* FF33 */
}

<div class="grid">
  <div class="head">Column 1 (3 items)</div>
  <div class="data">item 1-1</div>
  <div class="data">item 1-2</div>
  <div class="data">item 1-3</div>
  <div class="head">Column 2 (4 items)</div>
  <div class="data">item 2-1</div>
  <div class="data">item 2-2</div>
  <div class="data">item 2-3</div>
  <div class="data">item 2-4</div>
  <div class="head">Column 3 (2 items)</div>
  <div class="data">item 3-1</div>
  <div class="data">item 3-2</div>
  <div class="head">Column 4 (1 items)</div>
  <div class="data">item 4-1</div>
</div>