Update page title to indicate editing flows

frontend/src/main.js

@@ -15,9 +15,12 @@ import PageEditor from './PageEditor.vue'
 import SiteEditor from './SiteEditor.vue'
 
 let host = null
+let app = null
+let savedTitle = null
 
 export function openEditor(path, { mode = 'page' } = {}) {
   closeEditor()
+  savedTitle = document.title
   host = document.createElement('div')
   host.className = 'editor-host'
   // Docked inside #content: below the banner, next to the article only.
@@ -26,10 +29,11 @@ export function openEditor(path, { mode = 'page' } = {}) {
   // Which kind of editor is open; pagerite.js uses this to decide
   // whether a pen click closes the panel or swaps in the other editor.
   document.body.dataset.editorMode = mode
-  createApp(mode === 'site' ? SiteEditor : PageEditor, {
+  app = createApp(mode === 'site' ? SiteEditor : PageEditor, {
     pagePath: path,
     onClose: closeEditor,
-  }).mount(host)
+  })
+  app.mount(host)
   // The slide-in (editor-slide-in in pagerite.css) is a one-shot open
   // effect; once finished, drop it so that later stylesheet swaps (theme
   // change re-creating @keyframes) cannot restart it.
@@ -48,6 +52,26 @@ export function closeEditor() {
   // Slide the panel out in sync with the page shifting back.
   host.firstElementChild?.classList.add('closing')
   const old = host
+  const oldApp = app
+  const restoreTitle = savedTitle
   host = null
-  setTimeout(() => old.remove(), 250)
+  app = null
+  savedTitle = null
+  setTimeout(() => {
+    oldApp?.unmount()
+    old.remove()
+    if (host) return // a new editor has opened; its title is authoritative
+    // Restore the server-rendered title for the current URL. Re-fetching makes
+    // sure a brand change in the site editor or an in-place navigation leaves
+    // the correct public title behind.
+    fetch(location.pathname)
+      .then((r) => r.text())
+      .then((html) => {
+        const doc = new DOMParser().parseFromString(html, 'text/html')
+        if (doc.title) document.title = doc.title
+      })
+      .catch(() => {
+        if (restoreTitle != null) document.title = restoreTitle
+      })
+  }, 250)
 }