Plan-mode responses to steering messages disappear from the conversation #5160

Describe the bug

In assisted approval mode, when steering the conversation or asking a question during plan mode, the CLI's response may briefly appear and then disappear from the screen. The agent continues with a follow-up that refers to the missing explanation, but the explanation is no longer available in the conversation, even after expanding the Thought for line. Users may have to ask the question again.

Affected version

N/A — not provided.

Steps to reproduce the behavior

1. Start a session in assisted approval mode and enter plan mode.
2. While the plan is in progress, send a steering message or ask a question.
3. Observe whether the CLI's response appears briefly, then disappears as the agent continues.
4. Expand the associated Thought for line and check whether the response is available there.

This behavior was reported, but a consistent reproduction has not been confirmed.

Expected behavior

Messages and responses exchanged while steering the plan should remain visible and accessible in the conversation, including after the agent references them in a later response.

Additional context

Related but distinct reports: #4990 describes user messages being swallowed while the CLI is busy; #1238 describes plan-mode questions becoming hidden after choosing to answer all. Neither describes an assistant response to a steering message briefly appearing and then disappearing from conversation history.