Bug Description

When using the webchat interface, long responses with block streaming enabled cause a rendering bug where:

1. Content streams out normally for several lines
2. Cursor jumps back to the start of the response frame
3. New content overwrites the beginning of the response
4. User loses the first portion of the message

Steps to Reproduce

1. Use webchat interface (not Telegram)
2. Ask a question that generates a long response (multiple paragraphs)
3. Watch as content streams
4. Observe cursor jump and content overwrite

Expected Behavior

Block-streamed content should append without overwriting previously rendered text.

Impact

• Critical information loss for users
• Makes webchat unreliable for lengthy responses
• Forces users to switch to alternative interfaces (Telegram)

Workaround

User can switch to Telegram bot for reliable message delivery.

Environment

• Gateway mode: local
• Channel: webchat
• Block streaming: enabled (default)
• Session: agent:main:main (Sonnet 4.5)

Reported by user during active session on 2026-01-25.