Root cause: stream loop emitted a bare st.empty() heartbeat per tick, accumulating
dozens of empty slots under chat_message. On the next rerun the history replay
path had no such slots, so Streamlit's incremental DOM diff misaligned and left
the previous render of the last text block as a grayed-out ghost.
Fix: unify streaming and history replay under the same shell
(slot = st.empty(); with slot.container(): render_segments(...)). Heartbeat now
re-enters the same slot each tick; when response is unchanged Streamlit's diff
is a no-op (no flicker), but the container() call still lets StopException
propagate so abort keeps working. Simplified render_segments from 5 params
(placeholders/rendered_cache/force_text) to 2 (segments, suffix).
Net: +19 -20 lines. Verified: no ghost, abort interrupts mid-stream, per-turn
fold-collapse preserved.
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