Accepted answer

You can either set a non-relative width or flex-basis on the .main-panel container to help Highcharts figure out the parent element's dimensions.

I've also removed the width: 100% on .highcharts-container since it wasn't doing anything.

The original Plunkr works on Safari 9.0.1, so this could be due to differences in how browsers implement the flexbox spec (e.g. related issue with width/height: 100%)

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