垂直调整3个div中的1个以适应窗口?

html,
body {
  font: 13px Verdana;
  margin: 0;
  height: 100%;
}

#container {
  max-height: 1000px;
  max-width: 1200px;
  background: red;
  color: #fff;
  height: 100%;
  position: relative;
}

#vertically-static-top,
#vertically-static-bottom,
#vertically-resizeable-middle {
  position: absolute;
  left: 0;
  right: 0;
}

#vertically-static-top {
  height: 20px;
  top: 0;
  background: black;
}

#vertically-static-bottom {
  height: 50px;
  bottom: 0;
  background: black;
}

#vertically-resizeable-middle {
  top: 20px;
  bottom: 50px;
}

<div id="container">
  <div id="vertically-static-top">20 pixes high</div>
  <div id="vertically-resizeable-middle">adjustable height</div>
  <div id="vertically-static-bottom">50 pixels high</div>
</div>

#container {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical!important;
  -webkit-box-direction: normal!important;
  -ms-flex-direction: column!important;
  flex-direction: column!important;
  height: 240px;
  margin: auto;
  max-height: 1000px;
  max-width: 1200px;
}

#vertically-static-top,
#vertically-static-bottom {
  background-color: #80bdff;
  border-radius: .25rem;
  color: #fff;
  padding: 1rem;
}

#vertically-resizeable-middle {
  background-color: #957bbe;
  border-radius: .25rem;
  color: #fff;
  flex-grow: 1;
  padding: 1rem;
}

<div id="container"> maximum 1200 width and 1000 height
    <div id="vertically-static-top">20 pixes high</div>
    <div id="vertically-resizeable-middle">adjustable height</div>
    <div id="vertically-static-bottom">50 pixels high</div>
</div>