Expand nav to current page on direct URL load

Walk up the DOM from the matched link and check each ancestor
collapsible checkbox so the sidebar reveals the active page location
when navigating directly to a URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: theletterf

src/Elastic.Documentation.Site/Assets/pages-nav-v2.ts

@@ -42,8 +42,6 @@ function initAccordion(nav: HTMLElement) {
 
 /**
  * Mark the current page's nav link with the "current" CSS class.
- * Unlike the V1 nav, we do NOT auto-expand parent sections —
- * progressive disclosure keeps all sections collapsed until the user opens them.
  */
 function markCurrentPage(nav: HTMLElement) {
     // Remove stale current markers
@@ -55,11 +53,35 @@ function markCurrentPage(nav: HTMLElement) {
     )
 }
 
+/**
+ * Expand all ancestor collapsible sections that contain the current page link,
+ * so that navigating directly to a URL reveals its location in the sidebar.
+ */
+function expandToCurrentPage(nav: HTMLElement) {
+    const pathname = window.location.pathname.replace(/\/$/, '')
+    const link = nav.querySelector<HTMLElement>(
+        `a[href="${pathname}"], a[href="${pathname}/"]`
+    )
+    if (!link) return
+
+    let el: Element | null = link.parentElement
+    while (el && el !== nav) {
+        if (el.matches('li')) {
+            const cb = el.querySelector<HTMLInputElement>(
+                ':scope > .peer input[type=checkbox]'
+            )
+            if (cb) cb.checked = true
+        }
+        el = el.parentElement
+    }
+}
+
 /**
  * Initialize all V2 nav behaviours on the given sidebar element.
  * Call this on every htmx:load when [data-nav-v2] is present.
  */
 export function initNavV2(nav: HTMLElement) {
     initAccordion(nav)
     markCurrentPage(nav)
+    expandToCurrentPage(nav)
 }