fix(stapp): own stream bubble inside fragment for cross-version rerun safety

frontends/stapp.py

@@ -434,20 +434,20 @@ if prompt:
     with st.chat_message("user"): st.markdown(prompt)
     _start_main_task(prompt)
 
-# Stream hosts only when this session owns the queue.
-# Poll quickly while active; reduce idle renderer churn.
-_stream_fh = None
-if st.session_state.get('display_queue') is not None:
-    with st.chat_message("assistant"):
-        _stream_fh = st.container()
-elif agent.is_running:
+# Stream bubble is owned by the fragment below: a fragment atomically replaces
+# its *own* subtree on every rerun in all Streamlit versions, whereas writing
+# into a container created outside the fragment is version-dependent (pre-1.62
+# appends forever → duplicates; ≥1.62 resets/GCs → disappears). So never hoist
+# the host out of the fragment; paint the full desired state each tick.
+_owns_stream = st.session_state.get('display_queue') is not None
+if not _owns_stream and agent.is_running:
     st.chat_message("assistant").markdown(T("detached_running"))
 
-@st.fragment(run_every=timedelta(seconds=1 if (_stream_fh is not None or agent.is_running or st.session_state.get('loop_enabled')) else 5))
+@st.fragment(run_every=timedelta(seconds=1 if (_owns_stream or agent.is_running or st.session_state.get('loop_enabled')) else 5))
 def _tick():
     """Poll every second while active and every five seconds while idle."""
     # 1) Own stream: drain done, paint all_outputs
-    if _stream_fh is not None:
+    if _owns_stream:
         done = _poll_main_task()
         if done is not None:
             if done:
@@ -466,16 +466,12 @@ def _tick():
         _dq = st.session_state.get("display_queue")
         steps = (list(((agent.all_outputs or [{}])[-1].get("outputs")) or [])
                  if _dq is getattr(agent, "_current_queue", None) else [])
-        # Streamlit ≥1.62: a fragment's outside-container wrapper resets to
-        # index 0 on every fragment rerun and children not re-emitted are GC'd
-        # (see runtime/fragment.py:_reset_outside_wrappers). Incremental appends
-        # don't survive across ticks, so repaint *everything* every tick — frozen
-        # expanders plus the live tail — into the single host container. Always
-        # writing (even when steps is empty) also claims the slot on the initial
-        # run, avoiding "could not reserve a stable position".
         live = re.sub(r'\**LLM Running \(Turn \d+\) \.\.\.\**\s*$', '',
                       (steps[-1] if steps else '') or '').rstrip()
-        with _stream_fh:
+        # Idempotent repaint inside the fragment's own subtree: the whole
+        # bubble (expanders + live tail) is re-emitted from state every tick,
+        # so a rerun can neither drop nor duplicate elements.
+        with st.chat_message("assistant"):
             for i in range(max(0, len(steps) - 1)):
                 body = steps[i] or ''
                 with st.expander(_step_title(body, i), expanded=False): st.markdown(body)