feat(theme-default): subtitle automatically appears in the view when it overflows the visual area

packages/@vuepress/theme-default/src/client/components/SidebarItems.vue

@@ -1,8 +1,46 @@
 <script setup lang="ts">
 import SidebarItem from '@theme/SidebarItem.vue'
+import { watch } from 'vue'
+import { useRoute } from 'vue-router'
 import { useSidebarItems } from '../composables'
 
+const route = useRoute()
 const sidebarItems = useSidebarItems()
+
+watch(
+  () => route.hash,
+  (hash) => {
+    // get the sidebar DOM
+    const sidebar = document.querySelector('.sidebar')
+    if (!sidebar) return
+
+    // get the active sidebar item DOM, whose href equals to the current route
+    const activeSidebarItem = document.querySelector(
+      `.sidebar a.sidebar-item[href="${route.path}${hash}"]`
+    )
+    if (!activeSidebarItem) return
+
+    // get the top and height of the sidebar
+    const sidebarTop = sidebar.getBoundingClientRect().top
+    const sidebarHeight = sidebar.getBoundingClientRect().height
+
+    // get the top and height of the active sidebar item
+    const activeSidebarItemTop = activeSidebarItem.getBoundingClientRect().top
+    const activeSidebarItemHeight =
+      activeSidebarItem.getBoundingClientRect().height
+
+    if (activeSidebarItemTop < sidebarTop) {
+      // scroll to the top edge of sidebar when the active sidebar item overflows the top edge of sidebar
+      activeSidebarItem.scrollIntoView(true)
+    } else if (
+      activeSidebarItemTop + activeSidebarItemHeight >
+      sidebarTop + sidebarHeight
+    ) {
+      // scroll to the bottom edge of sidebar when the active sidebar item overflows the bottom edge of sidebar
+      activeSidebarItem.scrollIntoView(false)
+    }
+  }
+)
 </script>
 
 <template>