fix($default-theme): sidebar groups are not opened when directly navi…

…gating to these pages (fix #2564) (#2565)

packages/@vuepress/theme-default/components/SidebarLinks.vue

@@ -90,13 +90,16 @@ function resolveOpenGroupIndex (route, items) {
 
 function descendantIsActive (route, item) {
   if (item.type === 'group') {
-    return item.children.some(child => {
+    const childIsActive = item.path && isActive(route, item.path)
+    const grandChildIsActive = item.children.some(child => {
       if (child.type === 'group') {
         return descendantIsActive(route, child)
       } else {
         return child.type === 'page' && isActive(route, child.path)
       }
     })
+
+    return childIsActive || grandChildIsActive
   }
   return false
 }