diff --git a/frontend/src/features/detail/DetailPage.tsx b/frontend/src/features/detail/DetailPage.tsx
index ba269ea..fc772b9 100644
--- a/frontend/src/features/detail/DetailPage.tsx
+++ b/frontend/src/features/detail/DetailPage.tsx
@@ -14,11 +14,14 @@ export default function DetailPage() {
   if (J_ID_RE.test(itemId)) {
     if (!hasBackend()) return <BackendNeeded />;
     const normalized = `J-${itemId.slice(2)}`;
-    return <LibraryDetail jId={normalized} />;
+    // Key per item: remount so local state (delete confirm, optimizer, task
+    // view) cannot leak from the previously viewed item.
+    return <LibraryDetail key={normalized} jId={normalized} />;
   }
 
   if (/^\d+$/.test(itemId)) {
-    return <OnlineDetail postId={Number(itemId)} />;
+    // Key per post: the download task view must not survive a post change.
+    return <OnlineDetail key={itemId} postId={Number(itemId)} />;
   }
 
   return (
diff --git a/frontend/src/features/online/OnlineDetail.tsx b/frontend/src/features/online/OnlineDetail.tsx
index 44c52dc..de164b5 100644
--- a/frontend/src/features/online/OnlineDetail.tsx
+++ b/frontend/src/features/online/OnlineDetail.tsx
@@ -121,6 +121,10 @@ export function OnlineDetail({ postId }: { postId: number }) {
   });
 
   const [downloadTaskId, setDownloadTaskId] = useState<string | null>(null);
+  // True once this visit has seen the download running. Only then is "your
+  // last download finished" worth showing; a revisit stays quiet (the "In
+  // library" button already says where the file is).
+  const [watchedDownload, setWatchedDownload] = useState(false);
 
   // Server-side tasks outlive this page; re-attach to the newest one for this
   // post so navigating away and back keeps the progress view.
@@ -186,6 +190,19 @@ export function OnlineDetail({ postId }: { postId: number }) {
   const downloadStatus = downloadTask?.status;
   const downloadActive =
     downloadStatus === "pending" || downloadStatus === "downloading";
+  // Never render another post's task — this covers the single frame before the
+  // remount (DetailPage keys this component per post). A null post_id (older or
+  // hand-made tasks) stays visible rather than hiding a real download.
+  const downloadTaskMatchesPost =
+    !downloadTask ||
+    downloadTask.post_id === null ||
+    downloadTask.post_id === post?.id;
+
+  // Derived state (safe to set during render): remember that this visit saw
+  // the download running.
+  if (downloadActive && !watchedDownload) {
+    setWatchedDownload(true);
+  }
 
   useEffect(() => {
     if (!downloadStatus) return;
@@ -340,7 +357,8 @@ export function OnlineDetail({ postId }: { postId: number }) {
             </a>
           </div>
 
-          {downloadTask &&
+          {downloadTaskMatchesPost &&
+          downloadTask &&
           (downloadTask.status === "pending" ||
             downloadTask.status === "downloading") ? (
             <div className="rounded-lg border border-ctp-surface0 bg-ctp-base p-3">
@@ -372,7 +390,7 @@ export function OnlineDetail({ postId }: { postId: number }) {
                 cancel
               </button>
             </div>
-          ) : downloadTask?.status === "complete" ? (
+          ) : downloadTaskMatchesPost && downloadTask?.status === "complete" ? (
             <p className="flex items-center gap-2 rounded-md border border-ctp-green/30 bg-ctp-green/10 px-3 py-2 text-sm text-ctp-green">
               <CheckCircle2 className="h-4 w-4 shrink-0" />
               Downloaded to the library
@@ -385,13 +403,13 @@ export function OnlineDetail({ postId }: { postId: number }) {
                 </Link>
               ) : null}
             </p>
-          ) : downloadTask?.status === "cancelled" ? (
+          ) : downloadTaskMatchesPost && downloadTask?.status === "cancelled" ? (
             <p className="text-xs text-ctp-overlay0">Download cancelled.</p>
-          ) : downloadTask?.status === "error" ? (
+          ) : downloadTaskMatchesPost && downloadTask?.status === "error" ? (
             <p className="rounded-md border border-ctp-red/30 bg-ctp-red/10 px-3 py-2 text-sm text-ctp-red">
               {downloadTask.error || "Download failed."}
             </p>
-          ) : remoteFinishedTask && !downloadMutation.isPending ? (
+          ) : remoteFinishedTask && watchedDownload && !downloadMutation.isPending ? (
             <p className="flex items-center gap-2 text-xs text-ctp-overlay0">
               <CheckCircle2 className="h-3.5 w-3.5 shrink-0 text-ctp-green" />
               Your last download for this post finished