style(gatsby-theme-docz): keep NavGroup opened on item click (#1211)

Author: Kerumen

core/gatsby-theme-docz/src/components/NavGroup/index.js

@@ -1,14 +1,18 @@
 /** @jsx jsx */
 import { jsx } from 'theme-ui'
 import React from 'react'
+import { useCurrentDoc } from 'docz'
 
 import * as styles from './styles'
 import { NavLink } from '../NavLink'
 import { ChevronDown } from '../Icons'
 
 export const NavGroup = ({ item }) => {
-  const { menu } = item
-  const [subheadingsVisible, setShowsubheadings] = React.useState(false)
+  const current = useCurrentDoc()
+  const { name, menu } = item
+  const [subheadingsVisible, setShowsubheadings] = React.useState(
+    current.menu === name
+  )
   const toggleSubheadings = () => setShowsubheadings(!subheadingsVisible)
 
   return (