Flexbox Sidebar

Create a sidebar layout with flexbox that automatically stacks when the container is narrow.

<div
  sx={{
    display: 'flex',
    flexWrap: 'wrap',
  }}>
  <aside
    sx={{
      flexGrow: 1,
      flexBasis: 'sidebar',
    }}>
    Sidebar
  </aside>
  <main
    sx={{
      flexGrow: 99999,
      flexBasis: 0,
      minWidth: 320,
    }}>
    Main
  </main>
</div>
<div
  sx={{
    display: 'flex',
    flexWrap: 'wrap',
  }}>
  <aside
    sx={{
      flexGrow: 1,
      flexBasis: 'sidebar',
    }}>
    Sidebar
  </aside>
  <main
    sx={{
      flexGrow: 99999,
      flexBasis: 0,
      minWidth: 320,
    }}>
    Main
  </main>
</div>

// example theme
export default {
  sizes: {
    sidebar: 256,
  },
}

/** @jsx jsx */
import { jsx } from 'theme-ui'
export default props =>
  <div
    sx={{
      display: 'flex',
      flexWrap: 'wrap',
    }}>
    <aside
      sx={{
        flexGrow: 1,
        flexBasis: 'sidebar',
      }}>
      Sidebar
    </aside>
    <main
      sx={{
        flexGrow: 99999,
        flexBasis: 0,
        minWidth: 320,
      }}>
      {props.children}
    </main>
  </div>