fix: make the browser Back button close a post instead of exiting the app

The URL was only ever synced with history.replaceState, so the app
never created any history entries of its own -- Back always went
straight to whatever page was open before this one, no matter where
you were in the app.

Opening a post now pushState's a new entry, matching how Instagram's
own back button behaves: Back closes the post and returns to the grid.
Closing a post any other way (the X button, the modal's own close
handler) consumes that same entry via history.back() instead of piling
a fresh one on top, and a popstate listener re-syncs app state for
both directions. Tab switches and archive loads still use
replaceState, unchanged -- only the post view gets its own step in
history, deliberately, to keep the history stack shallow.

Verified in a real browser: open a post, Back closes it and stays in
the app; Forward reopens it; the X button closes it too, consuming the
same entry rather than leaving a stale one behind.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011qAds5qr7nZRq5R4yAuxUk

src/App.tsx

@@ -73,6 +73,17 @@ export default function App() {
    */
   const initialRouteRef = useRef(parseRoute(window.location.pathname, window.location.search));
 
+  /**
+   * Back-button support for the post view. Every other URL change
+   * (`replaceState`s the tab/archive) is intentionally NOT pushed — only
+   * opening a post gets its own history entry, matching Instagram's own
+   * back-button behaviour: Back closes the post instead of leaving the app.
+   */
+  const pushedPostRef = useRef(false);
+  /** Set while reacting to a popstate, so the URL-sync effect below does not
+   * try to push/replace/back() again for a change the browser already made. */
+  const suppressNextSyncRef = useRef(false);
+
   const isMobile = useIsMobile();
   const fileInputRef = useRef<HTMLInputElement>(null);
   const profilePicInputRef = useRef<HTMLInputElement>(null);
@@ -367,6 +378,13 @@ export default function App() {
     // loader below is waiting to read.
     if (!hasInitialLoaded) return;
 
+    // Consumed exactly once per popstate, regardless of what happens below:
+    // a popstate-driven change often already matches the URL (the browser
+    // already moved the pointer), which used to leave this flag stuck true
+    // and silently no-op the NEXT real close/open until something reset it.
+    const wasPopState = suppressNextSyncRef.current;
+    suppressNextSyncRef.current = false;
+
     const archive = currentArchive?.name ?? (allPosts.length > 0 ? username : null) ?? null;
     const nextPath = buildPath({
       archive,
@@ -374,12 +392,48 @@ export default function App() {
       post: selectedPost ? postSlug(selectedPost) : null,
     });
 
-    if (nextPath !== window.location.pathname + window.location.search) {
+    if (nextPath === window.location.pathname + window.location.search) return;
+    if (wasPopState) return; // the browser already navigated; nothing to add
+
     console.log(`[Permalink] Updating URL to: ${nextPath}`);
+
+    if (selectedPost && !pushedPostRef.current) {
+      // Opening a post: push, so Back closes it instead of leaving the app.
+      window.history.pushState(null, '', nextPath);
+      pushedPostRef.current = true;
+    } else if (!selectedPost && pushedPostRef.current) {
+      // Closing a post that was pushed for: consume that entry rather than
+      // piling a new one on top of it, so Back still means "one step".
+      pushedPostRef.current = false;
+      window.history.back();
+    } else {
       window.history.replaceState(null, '', nextPath);
     }
   }, [hasInitialLoaded, currentArchive?.name, username, allPosts.length, activeTab, selectedPost?.id]);
 
+  /**
+   * Back/forward support for the post view. Only a post push (above) ever
+   * creates an entry, so this only ever needs to open or close a post —
+   * never re-derive the tab or archive, which stayed on replaceState.
+   */
+  useEffect(() => {
+    const onPopState = () => {
+      suppressNextSyncRef.current = true;
+      const route = parseRoute(window.location.pathname, window.location.search);
+      const post = route.post ? findPostBySlug(allPosts, route.post) : null;
+      if (post) {
+        setActiveTab(tabForSource(post.source));
+        setSelectedPost(post);
+        pushedPostRef.current = true; // forward navigation can land back here
+      } else {
+        setSelectedPost(null);
+        pushedPostRef.current = false;
+      }
+    };
+    window.addEventListener('popstate', onPopState);
+    return () => window.removeEventListener('popstate', onPopState);
+  }, [allPosts]);
+
   useEffect(() => {
     if (hasInitialLoaded) return;