Assistant text replies after mid-turn user messages silently dropped by terminal renderer (rendered as bare '[')

## What happened

During a long working session with background workflows running, I sent several mid-turn messages. The assistant's text replies to at least two of them were **never displayed in the terminal** — the transcript shows my message, then a stray `[` character on its own line, then the assistant's next tool call. No reply text.

The assistant *did* reply: the session JSONL on disk contains the full text block (verified by grep — exactly one occurrence, correctly sequenced after my message). Only the rendering dropped it.

## Why it's serious

From the user's side this is indistinguishable from the agent ignoring you. I escalated twice ("why are you ignoring me?") against an agent that was in fact answering every message. A renderer bug that manufactures the "my agent ignores me" experience is trust-destroying in a way ordinary display glitches aren't — and it's only detectable if you later notice a reply referencing text you never saw.

## Repro context

- Long turn with many tool calls; user message arrives mid-turn (surfaced alongside a tool result).
- Assistant emits a text block **first**, then tool calls (Edit/Bash) in the same response.
- The text block renders as a single `[` character; the tool calls after it render normally.
- Happened at least twice in one session (2026-07-25). Screenshots exist showing the orphaned `[` directly under the user's message.
- Session conditions: multiple background Workflow tasks streaming progress; `/workflows` monitor opened earlier in the session; terminal 264×67; `--dangerously-skip-permissions`; macOS (Darwin 27).
- Version: Claude Code 2.1.220, model claude-fable-5.

## Expected

The text block renders before the tool calls — or at minimum is recoverable in the transcript view (ctrl+o did not surface it).

## Suspected area

Race between mid-turn user-message injection and assistant text-block rendering when tool calls follow in the same message — the orphaned `[` suggests the block's opening delimiter rendered before the content was dropped.

## Related smaller issue

The workflow monitor shows `idle Ns` for subagents that are mid-generation (it only tracks tool-call activity). "analyzing" / "no tool activity Ns" would be honest without implying a hang — `idle 43s` on a synthesis agent reads as hung and generates exactly the confusion above.

Reproduce a long turn with a mid-turn user message, background Workflow activity, and an assistant text block followed by tool calls; compare terminal output with the session JSONL on disk. Check the ctrl+o transcript view and workflow monitor while testing. Done means the full text renders before the tool calls or is recoverable in the transcript, without an orphaned '['.

Written by the indexing model from the issue text.