CSS Flex Box Layout: full-width row and columns

Hello fellow programmers!

I've got a simple box-layout which I would love to achieve using flexbox, but I simply can't figure it out. It should look like this image.

So basically a row and two columns, with the row being fixed at lets say 100px in height, but all in one container. My code so far is:

#productShowcaseContainer {
  display: inline-flex;
  flex-flow: row wrap;
  height: 600px;
  width: 580px;
  background-color: rgb(240, 240, 240);
}

#productShowcaseTitle {
  display: inline-block;
  height: 100px;
  width: 100%;
  background-color: rgb(200, 200, 200);
}

#productShowcaseDetail {
  flex: 3;
  background-color: red;
}

#productShowcaseThumbnailContainer {
  flex: 2;
  background-color: blue;
}

<div id="productShowcaseContainer">
  <div id="productShowcaseTitle"></div>
  <div id="productShowcaseDetail"></div>
  <div id="productShowcaseThumbnailContainer"></div>
</div>

I know this can be achieved in many ways, but I would really prefer to use CSS Flex.

You've almost done it. However setting flex: 0 0 <basis> declaration to the columns would prevent them from growing/shrinking; And the <basis> parameter would define the width of columns.

In addition, you could use CSS3 calc() expression to specify the height of columns with the respect to the height of the header.

#productShowcaseTitle {
  flex: 0 0 100%; /* Let it fill the entire space horizontally */
  height: 100px;
}

#productShowcaseDetail,
#productShowcaseThumbnailContainer {
  height: calc(100% - 100px); /* excluding the height of the header */
}

#productShowcaseContainer {
  display: flex;
  flex-flow: row wrap;

  height: 600px;
  width: 580px;
}

#productShowcaseTitle {
  flex: 0 0 100%; /* Let it fill the entire space horizontally */
  height: 100px;
  background-color: silver;
}

#productShowcaseDetail {
  flex: 0 0 66%; /* ~ 2 * 33.33% */
  height: calc(100% - 100px); /* excluding the height of the header */
  background-color: lightgray;
}

#productShowcaseThumbnailContainer {
  flex: 0 0 34%;  /* ~ 33.33% */
  height: calc(100% - 100px); /* excluding the height of the header */
  background-color: black;
}

<div id="productShowcaseContainer">
  <div id="productShowcaseTitle"></div>
  <div id="productShowcaseDetail"></div>
  <div id="productShowcaseThumbnailContainer"></div>
</div>

(Vendor prefixes omitted due to brevity)

---

Alternatively, if you could change your markup e.g. wrapping the columns by an additional <div> element, it would be achieved without using calc() as follows:

<div class="contentContainer"> <!-- Added wrapper -->
    <div id="productShowcaseDetail"></div>
    <div id="productShowcaseThumbnailContainer"></div>
</div>

#productShowcaseContainer {
  display: flex;
  flex-direction: column;
  height: 600px; width: 580px;
}

.contentContainer { display: flex; flex: 1; }
#productShowcaseDetail { flex: 3; }
#productShowcaseThumbnailContainer { flex: 2; }

#productShowcaseContainer {
  display: flex;
  flex-direction: column;

  height: 600px;
  width: 580px;
}

.contentContainer {
  display: flex;
  flex: 1;
}

#productShowcaseTitle {
  height: 100px;
  background-color: silver;
}

#productShowcaseDetail {
  flex: 3;
  background-color: lightgray;
}

#productShowcaseThumbnailContainer {
  flex: 2;
  background-color: black;
}

<div id="productShowcaseContainer">
  <div id="productShowcaseTitle"></div>

  <div class="contentContainer"> <!-- Added wrapper -->
    <div id="productShowcaseDetail"></div>
    <div id="productShowcaseThumbnailContainer"></div>
  </div>
</div>

(Vendor prefixes omitted due to brevity)

Just use another container to wrap last two divs. Don't forget to use CSS prefixes.

#productShowcaseContainer {
   display: flex;
   flex-direction: column;
   height: 600px;
   width: 580px;
   background-color: rgb(240, 240, 240);
}

#productShowcaseTitle {
   height: 100px;
   background-color: rgb(200, 200, 200);
}

#anotherContainer{
   display: flex;
   height: 100%;
}

#productShowcaseDetail {
   background-color: red;
   flex: 4;
}

#productShowcaseThumbnailContainer {
   background-color: blue;
   flex: 1;
}

<div id="productShowcaseContainer">
   <div id="productShowcaseTitle">1</div>
   <div id="anotherContainer">
      <div id="productShowcaseDetail">2</div>
      <div id="productShowcaseThumbnailContainer">3</div>
   </div>
</div>

This is copied from above, but condensed slightly and re-written in semantic terms. Note: #Container has display: flex; and flex-direction: column;, while the columns have flex: 3; and flex: 2; (where "One value, unitless number" determines the flex-grow property) per MDN flex docs.

#Container {
  display: flex;
  flex-direction: column;
  height: 600px;
  width: 580px;
}

.Content {
  display: flex;
  flex: 1;
}

#Detail {
  flex: 3;
  background-color: lime;
}

#ThumbnailContainer {
  flex: 2;
  background-color: black;
}

<div id="Container">
  <div class="Content">
    <div id="Detail"></div>
    <div id="ThumbnailContainer"></div>
  </div>
</div>