Public language selector, linked with the editor's language selection. Shared popup menu operation with edit toolbar with consistent closing logic.

frontend/src/langselect-main.js

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+// Public language-selector entry: imported on demand by pagerite.js on
+// pages advertising more than one language in their hreflang alternates.
+// Vue, Pinia and the flag SVG set live in this chunk only — untranslated
+// pages never pay for them. The selector's state lives in the shared
+// store (./store), not the DOM: the corner container is rebuilt freely
+// and ensureMounted re-mounts from the store.
+import { createApp } from 'vue'
+import LangSelector from './LangSelector.vue'
+import { pinia, useStore } from './store'
+
+let app = null
+
+function store() {
+  return useStore(pinia)
+}
+
+// The current page's languages (called on every navigation).
+export function setLanguages(alternates, current) {
+  Object.assign(store(), {
+    langAlternates: alternates,
+    servedLang: current,
+    langSelectorActive: true,
+  })
+}
+
+// The current page is single-language: the selector goes away.
+export function hide() {
+  store().langSelectorActive = false
+  app?.unmount()
+  app = null
+}
+
+// Mount the selector as the container's first item; re-mount when its
+// element went away with a container rebuild (a live app updates from the
+// store reactively).
+export function ensureMounted(host) {
+  if (!store().langSelectorActive || !host) {
+    app?.unmount()
+    app = null
+    return
+  }
+  if (app && host.contains(app._container)) return
+  app?.unmount()
+  const el = document.createElement('div')
+  el.id = 'lang-selector'
+  host.prepend(el)
+  app = createApp(LangSelector)
+  app.use(pinia)
+  app.mount(el)
+}