[Solved]-Highcharts doesn't resize properly once expanded-Highcharts

If you want to achieve two equal boxes sets display:flex to parent and add children width: 50%.

.flex {
  display: flex;
}
#container {
  width: 50%;
}
.spacer {
  width: 50%;
  height: 500px;
  background-color: red;
}

Demo: https://jsfiddle.net/BlackLabel/46o7venk/