Unify navigation: URL-backed views, one shared history

Per audit (user + Codex): the in-page Back and browser Back were two separate
histories, which is confusing — especially for less-technical users. Make the
URL the single source of truth so both traverse one history.

* The view derives from the URL (/?view=latest, /?tag=, /?source=, bare / for
  Highlights); `selected` is $derived from $page.url.
* All navigation goes through goto(); afterNavigate is the single loader hook,
  so in-app clicks AND browser back/forward reload the same way.
* The in-page Back button now just calls history.back() (fallback to Highlights)
  — identical to the browser Back. Removed the private navStack.
* Stop stripping ?source= — the URL stays honest, so source/tag views are
  shareable and survive reload.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -1,6 +1,7 @@
 <script>
   import { onMount, untrack } from 'svelte';
-  import { goto } from '$app/navigation';
+  import { goto, afterNavigate } from '$app/navigation';
+  import { page } from '$app/stores';
   import { getJSON, postJSON } from '$lib/api.js';
   import * as P from '$lib/prefs.js';
   import Header from '$lib/components/Header.svelte';
@@ -20,8 +21,24 @@
   let families = $state([]);
   let lanePool = $state(null); // /api/lanes: { pinned, default, groups }
   let showLanes = $state(false);
-  let selected = $state('today'); // 'today' | mood | topic | 'tag:<slug>' | 'source:<id>'
+  // The URL is the single source of truth for the current view, so the in-page
-  let currentSource = $state(null); // {id, name} for a 'source:<id>' view's header
+  // Back button and the browser Back button share ONE history. The view is
+  // derived from query params: /?view=latest, /?tag=clean-energy, /?source=7,
+  // /?view=<mood|topic>, or bare / for Highlights.
+  function parseView(url) {
+    const p = url.searchParams;
+    if (p.get('source')) return 'source:' + p.get('source');
+    if (p.get('tag')) return 'tag:' + p.get('tag');
+    return p.get('view') || 'today';
+  }
+  function urlForView(key) {
+    if (key === 'today') return '/';
+    if (key.startsWith('source:')) return '/?source=' + encodeURIComponent(key.slice(7));
+    if (key.startsWith('tag:')) return '/?tag=' + encodeURIComponent(key.slice(4));
+    return '/?view=' + encodeURIComponent(key);
+  }
+  let selected = $derived(parseView($page.url));
+  let sourceNames = $state({}); // source id -> name, for an instant header label
   let brief = $state(null);
   let heroIdx = $state(0);
   let feed = $state([]);
@@ -74,7 +91,7 @@
     }
     loadServerHistory();
     await syncPrefsOnLogin(); // adopt account prefs or seed from device
-    select(selected, true); // reflect any adopted boundaries in the feed
+    loadView(selected, true); // reflect any adopted boundaries in the current view
   }
 
   const cap = (s) => (s ? s[0].toUpperCase() + s.slice(1) : s);
@@ -99,7 +116,7 @@
   );
   let viewLabel = $derived(
     selected === 'today' ? 'Highlights from Today'
-      : selected.startsWith('source:') ? (currentSource?.name ?? feed[0]?.source ?? 'Source')
+      : selected.startsWith('source:') ? (sourceNames[selected.slice(7)] ?? feed[0]?.source ?? 'Source')
       : selected === 'latest' ? 'Latest'
         : currentTag ? humanize(currentTag)
           : (currentMood?.label ?? cap(currentTopic?.key) ?? '')
@@ -140,7 +157,7 @@
     persistPrefs();
     // If the reader unpinned the lane they're viewing, fall back to Highlights.
     // (The pinned Highlights/Latest lanes are never in `keys`, so don't bounce.)
-    if (selected !== 'today' && selected !== 'latest' && !keys.includes(selected)) select('today');
+    if (selected !== 'today' && selected !== 'latest' && !keys.includes(selected)) navigate('today');
   }
 
   // Hero is the only image slot; if its image won't load, promote the next one.
@@ -208,27 +225,27 @@
     return `/api/feed?limit=${PAGE}&offset=${offset}${q ? '&' + q : ''}${exq}`;
   }
 
-  // Drill into a tag or source view from a card, remembering where we came from
+  // All navigation goes through the URL (goto), so browser Back/Forward and the
-  // so the in-feed Back button can return there (chains of drill-ins too).
+  // in-page Back button traverse the same single history.
-  let navStack = $state([]); // [{ key, source }] views to return to
+  function navigate(key) {
+    goto(urlForView(key));
+  }
+  // Drilling into a tag/source from a card: cache the source name for an instant
+  // label, then navigate by URL (no private stack).
   function drill(key, source = null) {
-    navStack = [...navStack, { key: selected, source: currentSource }];
+    if (source) sourceNames = { ...sourceNames, [source.id]: source.name };
-    if (source) currentSource = source;
+    navigate(key);
-    select(key);
   }
   function goBack() {
-    const prev = navStack[navStack.length - 1];
+    if (typeof history !== 'undefined' && history.length > 1) history.back();
-    if (!prev) return;
+    else goto(urlForView('today'));
-    navStack = navStack.slice(0, -1);
-    currentSource = prev.source;
-    select(prev.key);
   }
 
-  async function select(key, fresh = false) {
+  // Load the data for a view. Called by afterNavigate (URL-driven: in-app goto,
-    // Top-level navigation (nav rail / bottom bar) resets the drill-in history;
+  // browser back/forward, initial load) and directly on a boundary re-filter.
-    // only tag/source drill-ins build it up.
+  let loadSeq = 0;
-    if (!key.startsWith('tag:') && !key.startsWith('source:')) navStack = [];
+  async function loadView(key, fresh = false) {
-    selected = key;
+    const seq = ++loadSeq; // a newer navigation supersedes a slow in-flight one
     error = '';
     feedDone = false;
     try {
@@ -236,16 +253,27 @@
         await loadToday(fresh);
       } else {
         const items = (await getJSON(feedUrl(key, 0))).items;
+        if (seq !== loadSeq) return;
         feed = items;
         feedDone = items.length < PAGE;
         markDisplayed(feed);
+        if (key.startsWith('source:') && items[0]) {
+          sourceNames = { ...sourceNames, [key.slice(7)]: items[0].source };
+        }
       }
     } catch (e) {
-      error = 'Something went quiet — could not reach the feed.';
+      if (seq === loadSeq) error = 'Something went quiet — could not reach the feed.';
     }
     if (typeof window !== 'undefined') window.scrollTo({ top: 0, behavior: 'smooth' });
   }
 
+  // Every URL change (in-app goto, browser back/forward, link) reloads the view.
+  // The initial 'enter' is handled by onMount, after its data deps are fetched.
+  afterNavigate((nav) => {
+    if (nav.type === 'enter') return;
+    loadView(selected);
+  });
+
   // "Load more" for any feed view (Latest, topics, tags, moods): fetch the next
   // page at the current length and append, de-duping against what's shown.
   async function loadMore() {
@@ -269,7 +297,7 @@
   function applyAction(kind, value) {
     applyPrefAction(kind, value); // updates + persists + syncs to account
     if (MIX_EVENT[kind]) track(MIX_EVENT[kind]); // emotional-mix signal (no value stored)
-    select(selected, true); // re-filter the feed
+    loadView(selected, true); // re-filter the current view in place (no nav)
   }
 
   function flash(msg) {
@@ -319,10 +347,9 @@
     }
   }
 
-  function browse() {
+  async function browse() {
-    const go = () => document.getElementById('explore')?.scrollIntoView({ behavior: 'smooth' });
+    if (selected !== 'today') await goto(urlForView('today'));
-    if (selected !== 'today') select('today').then(go);
+    document.getElementById('explore')?.scrollIntoView({ behavior: 'smooth' });
-    else go();
   }
 
   onMount(async () => {
@@ -337,17 +364,9 @@
       topics = (await getJSON('/api/categories')).topics;
       try { lanePool = await getJSON('/api/lanes'); } catch { lanePool = null; }
       try { families = await getJSON('/api/families'); } catch { families = []; }
-      // Deep link from an article page's source name (/?source=<id>) → its feed.
+      // Load whatever the URL asks for (Highlights, or a deep-linked source/tag/
-      const srcParam = typeof location !== 'undefined' && new URLSearchParams(location.search).get('source');
+      // view) — the param stays in the URL so reload and share are honest.
-      if (srcParam && /^\d+$/.test(srcParam)) {
+      await loadView(selected);
-        await select('source:' + srcParam);
-        // Seed the back history so the in-feed Back returns to Highlights, and
-        // strip the ?source= param so it can't linger and confuse browser back.
-        navStack = [{ key: 'today', source: null }];
-        if (typeof history !== 'undefined') history.replaceState(null, '', location.pathname);
-      } else {
-        await select('today');
-      }
     } catch (e) {
       error = 'Could not reach Upbeat Bytes.';
     }
@@ -365,7 +384,7 @@
 
 <main class="container">
   {#if navLanes.length}
-    <MoodNav lanes={navLanes} {selected} onselect={select} oncustomize={() => (showLanes = true)} />
+    <MoodNav lanes={navLanes} {selected} onselect={navigate} oncustomize={() => (showLanes = true)} />
   {/if}
 
   {#if notice}<p class="notice rise">{notice}</p>{/if}
@@ -381,7 +400,7 @@
           <h1>{viewLabel}</h1>
           {#if viewSubtitle}<p class="sub">{viewSubtitle}</p>{/if}
         </div>
-        {#if navStack.length}
+        {#if selected !== 'today'}
           <button class="viewback" onclick={goBack} aria-label="Go back">
             <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M19 12H5M11 6l-6 6 6 6" fill="none" stroke="currentColor" stroke-width="2.1" stroke-linecap="round" stroke-linejoin="round"/></svg>
             Back
@@ -449,7 +468,7 @@
   {/if}
 </main>
 
-<BottomNav active={activeTab} onToday={() => select('today')} onLatest={() => select('latest')} onBrowse={browse} onYou={openAccount} user={auth.user} />
+<BottomNav active={activeTab} onToday={() => navigate('today')} onLatest={() => navigate('latest')} onBrowse={browse} onYou={openAccount} user={auth.user} />
 
 <style>
   main.container { padding-top: 6px; padding-bottom: 40px; min-height: 60vh; }