Flexbox with rotated text

Rotate the text but not the larger container. The header has 15px width, as specified.

.widget {
  height: 200px;
  width: 200px;
  border: 1px solid #ddd;
  background-color: #eee;
  display: flex;
}
.widget-header {
  background-color: #1976d2;
  flex: 0 0 15px;
  min-width: 0;
  display: flex;
  justify-content: center;
  align-items: center;
}
.widget-header > span {
  color: #fff;
  font-size: 10px;
  transform: rotate(-90deg);
  white-space: nowrap;
}
.widget-content {
  border: 1px solid blue;
  flex: 1;
}

<div class="widget">
  <div class="widget-header">
    <span>order summary</span>
  </div>
  <div class="widget-content">
    <p>some text here</p>
    <p>some more text ...</p>
  </div>
</div>

revised codepen

You will need to add one more wrap around rotated text. Then this newly inserted wrapper should be rotated instead of flex child.

HTML:

<div class="widget">
  <div class="widget-header">
    <div class="header-content">
      order summary
    </div>
  </div>
  <div class="widget-content">
    <p>some text here</p>
    <p>some more text ...</p>
  </div>
</div>

Necessary CSS:

.widget {
  position: relative;
  display: flex;
}
.widget-header {
  background-color: #1976D2;
  border: 1px solid red;
  text-align: center;
  color: #fff;
  width: 25px;
}
.header-content {
  transform: rotate(-90deg);
  transform-origin: 0 100%;
  position: absolute;
  line-height: 25px;
  font-size: 14px;
  height: 25px;
  width: 100%;
  left: 25px;
  bottom: 0;
}

* {box-sizing: border-box;}
.widget {
  height: 200px;
  width: 200px;
  border: 1px solid #ddd;
  background-color: #eee;
  position: relative;
  display: flex;
}
.widget-header {
  background-color: #1976D2;
  color: #fff;
  border: 1px solid red;
  font-size: 10px;
  width: 27px;
  text-align: center;
  height: 100%;
}
.header-content {
  transform: rotate(-90deg);
  transform-origin: 0 100%;
  position: absolute;
  line-height: 25px;
  font-size: 14px;
  z-index: 10;
  height: 25px;
  width: 100%;
  left: 25px;
  bottom: 0;
}
.widget-content {
  border: 1px solid blue;
  flex: 1;
}

<div class="widget">
  <div class="widget-header">
    <div class="header-content">
      order summary
    </div>
  </div>
  <div class="widget-content">
    <p>some text here</p>
    <p>some more text ...</p>
  </div>
</div>