fix(transcript): collapse a missed-reconcile prompt on session reentry

apps/desktop/src/lib/session-transcript.ts

@@ -202,9 +202,38 @@ export function mergeLiveSessionMessages(
   }
 
   const used = new Set<string>();
+  // An optimistic prompt whose reconcile event was missed survives with its
+  // temporary id, so id-based merging replays it next to the durable echo of
+  // the same prompt (#1308: prompt reappears after switching away and back).
+  // Each durable completed user row grants one collapse credit: an orphan
+  // live user row with equal text consumes one and is dropped. A genuinely
+  // repeated prompt keeps every durable row because two sends persist two
+  // rows while each live orphan still only consumes one credit.
+  const durableUserTextCredits = new Map<string, number>();
+  for (const message of durable) {
+    if (message.role !== "user") continue;
+    durableUserTextCredits.set(
+      message.content,
+      (durableUserTextCredits.get(message.content) ?? 0) + 1,
+    );
+  }
   const merged: UiMessage[] = [];
   const push = (message: UiMessage) => {
     if (used.has(message.id)) return;
+    if (
+      message.role === "user" &&
+      !isInFlightMessage(message) &&
+      !durableIds.has(message.id)
+    ) {
+      const credits = durableUserTextCredits.get(message.content) ?? 0;
+      if (credits > 0) {
+        durableUserTextCredits.set(message.content, credits - 1);
+        // Record the drop so a later merge pass cannot replay the orphan
+        // after its collapse credit has been consumed.
+        used.add(message.id);
+        return;
+      }
+    }
     used.add(message.id);
     merged.push(message);
   };
@@ -246,7 +275,19 @@ export function mergeLiveSessionMessages(
   }
 
   for (const message of liveNormalized) {
-    if (!used.has(message.id)) push(message);
+    if (used.has(message.id)) continue;
+    if (
+      message.role === "user" &&
+      !isInFlightMessage(message) &&
+      !durableIds.has(message.id)
+    ) {
+      const credits = durableUserTextCredits.get(message.content) ?? 0;
+      if (credits > 0) {
+        durableUserTextCredits.set(message.content, credits - 1);
+        continue;
+      }
+    }
+    push(message);
   }
 
   const unchanged =

apps/desktop/test/session-transcript.test.mjs

@@ -109,6 +109,58 @@ test("repeated transcript rows keep one position and the latest value", () => {
   );
 });
 
+test("an orphan optimistic prompt collapses into its durable echo (D334)", () => {
+  // The prompt was sent, the renderer switched away, and the reconcile event
+  // was missed: the optimistic row survives under its temporary id while the
+  // durable page carries the same prompt under the host id.
+  const orphan = message(
+    "11111111-2222-4333-8444-555555555555",
+    { role: "user", content: "1", createdAt: "2026-08-31T00:00:00.000Z" },
+  );
+  const echoed = message("sdk-user-1", {
+    role: "user",
+    content: "1",
+    createdAt: "2026-08-31T00:00:01.000Z",
+  });
+  const answer = message("sdk-answer-1", { content: "reply" });
+  const durable = [echoed, answer];
+  const live = [orphan, echoed, answer];
+
+  assert.deepEqual(
+    mergeLiveSessionMessages(durable, live).map(({ id, content }) => ({ id, content })),
+    [
+      { id: "sdk-user-1", content: "1" },
+      { id: "sdk-answer-1", content: "reply" },
+    ],
+  );
+});
+
+test("a genuinely repeated prompt stays visible after a switch", () => {
+  // Two real sends of the same text persist two durable rows; one live orphan
+  // may consume one, but the second repeat must survive the merge.
+  const orphan = message(
+    "11111111-2222-4333-8444-555555555555",
+    { role: "user", content: "1", createdAt: "2026-08-31T00:02:00.000Z" },
+  );
+  const first = message("sdk-user-1", {
+    role: "user",
+    content: "1",
+    createdAt: "2026-08-31T00:00:01.000Z",
+  });
+  const second = message("sdk-user-2", {
+    role: "user",
+    content: "1",
+    createdAt: "2026-08-31T00:02:01.000Z",
+  });
+  const durable = [first, second];
+  const live = [orphan, first, second];
+
+  assert.deepEqual(
+    mergeLiveSessionMessages(durable, live).map(({ id }) => id),
+    ["sdk-user-1", "sdk-user-2"],
+  );
+});
+
 test("live event upserts preserve array identity for unchanged rows", () => {
   const original = [message("answer", { status: "streaming" })];
   const updated = upsertLiveSessionMessage(original, {