fix(stream): require a current live assistant row to seed the replay floor

The replay-authority predicate accepted any non-empty assistant message,
but a new turn copies the full transcript into INFLIGHT.messages. In an
established conversation that content is the previous turn's reply, so
`canSeed` came back true with both stale cursor halves preserved and the
server kept skipping the range that rebuilds the body.

Message-based evidence now counts only from a current `_live` assistant
positioned after the latest user boundary; a historical assistant row
fails closed to floor 0 and an empty event id.

Refs #7640 (blocker on #7651)

static/sessions.js

@@ -1053,8 +1053,21 @@ function _inflightCanSeedJournalReplay(inflight){
   const anchorScene=inflight.anchorActivityScene;
   if(anchorScene&&Array.isArray(anchorScene.activity_rows)&&anchorScene.activity_rows.length) return true;
   if(Array.isArray(inflight.messages)){
-    return inflight.messages.some((msg)=>{
+    // The transcript copy carries every earlier turn (#7651): in an established
+    // conversation the last assistant row is the PREVIOUS turn's reply, not the
+    // one the cursor belongs to, so it must never authorize a nonzero floor.
+    // Message evidence counts only from a current `_live` assistant standing
+    // after the latest user boundary; anything historical fails closed to zero.
+    const list=inflight.messages;
+    let latestUserIdx=-1;
+    for(let i=list.length-1;i>=0;i--){
+      const row=list[i];
+      if(row&&row.role==='user'){latestUserIdx=i;break;}
+    }
+    return list.some((msg,idx)=>{
       if(!msg||msg.role!=='assistant') return false;
+      if(!msg._live) return false;
+      if(idx<=latestUserIdx) return false;
       const content=msg.content;
       if(typeof content==='string') return Boolean(content.trim());
       if(Array.isArray(content)) return content.length>0;

tests/test_inflight_stream_reuse.py

@@ -1113,7 +1113,7 @@ const recovering=[
   {toolCalls:[{name:'read_file'}]},
   {activityBurstAnchors:[{seq:12}]},
   {anchorActivityScene:{activity_rows:[{kind:'tool'}]}},
-  {messages:[{role:'user', content:'p'}, {role:'assistant', content:'answer'}]},
+  {messages:[{role:'user', content:'p'}, {role:'assistant', content:'answer', _live:true}]},
 ];
 for (const extra of recovering) {
   const state={messages:[{role:'user', content:'p'}], ...extra};
@@ -1127,6 +1127,39 @@ for (const extra of recovering) {
   assert.strictEqual(_runJournalReplayEventIdForInflight(state), 'stream-e75e:206');
 }
 
+// #7651: an ESTABLISHED conversation copies the whole transcript, so the last
+// assistant row is the previous turn's reply. Historical assistant content must
+// never authorize the stale cursor that belongs to the still-running turn.
+const established={
+  messages:[
+    {role:'user', content:'first question'},
+    {role:'assistant', content:'the previous turn answer'},
+    {role:'user', content:'current question'},
+  ],
+  lastRunJournalSeq:206,
+  lastRunJournalEventId:'stream-e75e:206',
+};
+assert.strictEqual(
+  _inflightCanSeedJournalReplay(established),
+  false,
+  'a historical assistant row must not seed the replay floor'
+);
+assert.strictEqual(_runJournalReplayFloorForInflight(established), 0);
+assert.strictEqual(_runJournalReplayEventIdForInflight(established), '');
+
+// Even after the latest user boundary, evidence must be LIVE output: a
+// settled/non-live assistant row at the current position still yields zero.
+assert.strictEqual(
+  _runJournalReplayFloorForInflight({
+    messages:[
+      {role:'user', content:'current question'},
+      {role:'assistant', content:'settled reply without _live'},
+    ],
+    lastRunJournalSeq:206,
+  }),
+  0
+);
+
 // Degenerate inputs must stay on the zero floor.
 assert.strictEqual(_runJournalReplayFloorForInflight(null), 0);
 assert.strictEqual(_runJournalReplayFloorForInflight({}), 0);
@@ -1193,6 +1226,29 @@ _normalizeInflightReplayCursorForReattach(recovering);
 assert.strictEqual(recovering.lastRunJournalSeq, 206);
 assert.strictEqual(recovering.lastRunJournalEventId, 'stream-e75e:206');
 
+// #7651: a historical assistant row must not protect the cursor either, or an
+// established conversation settles with a blank body over a painted footer.
+const established={
+  streamId:'stream-e75e',
+  reattach:true,
+  messages:[
+    {role:'user', content:'first question'},
+    {role:'assistant', content:'the previous turn answer'},
+    {role:'user', content:'current question'},
+  ],
+  lastRunJournalSeq:206,
+  lastRunJournalEventId:'stream-e75e:206',
+};
+_normalizeInflightReplayCursorForReattach(established);
+assert.strictEqual(established.lastRunJournalSeq, 0);
+assert.strictEqual(established.lastRunJournalEventId, '');
+assert.strictEqual(_runJournalReplayFloorForInflight(established), 0);
+assert.strictEqual(
+  established.messages.length,
+  3,
+  'the transcript must survive; only the cursor halves are dropped'
+);
+
 const noCursor={streamId:'stream-e75e', messages:[{role:'user', content:'p'}]};
 _normalizeInflightReplayCursorForReattach(noCursor);
 assert.strictEqual(noCursor.lastRunJournalSeq, undefined);