问为什么柔韧的东西在没有弯曲生长的情况下扩展全高度？

EN

.ai-container,
.ai-overlay {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  display: flex;
  justify-content: center;
}
.ai-overlay {
  background-color: black;
  opacity: .2;
  z-index: 1000;
}
.ai-dialog {
  z-index: 1001;
  display: flex;
  flex-direction: column;
  border: 1px solid darkgray;
  border-radius: 5px;
  background-color: white;
  min-width: 300px;
  box-shadow: 0px 0px 15px #444;
  overflow: hidden;
}
.ai-header,
.ai-footer,
.ai-body {
  padding: 10px;
}
.ai-header {
  background-color: hsl(0, 0%, 20%);
  color: white;
  display: flex;
  font-size: 1.1em;
  justify-content: space-between;
}
.ai-footer {
  display: flex;
  background-color: lightgray;
  justify-content: space-between;
}

<div class="ai-container">
  <div class="ai-overlay"></div>
  <div class="ai-dialog">
    <div class="ai-header">
      <span>This is the header</span>
      <span>X</span>
    </div>
    <div class="ai-body">This is the body</div>
    <div class="ai-footer">
      <span>
          <button>Submit</button>
          <button>Lookup</button>
      </span>
      <button>Cancel</button>
    </div>
  </div>
</div>

.ai-container, .ai-overlay {
    ... /*all the previous settings */
    align-items: center;
}