Frontend freezes when background Task agent completes, input buffer lost

Description

When a background agent (spawned via Task tool with run_in_background: true) completes, the Claude Code frontend freezes momentarily. Any text the user was typing in the input buffer at that moment is lost.

Steps to Reproduce

1. Start a Claude Code session
2. Spawn a background agent:

``
Task(
subagent_type="general-purpose",
run_in_background=true,
prompt="..."
)
``

1. While agent runs, start typing a long message in the input field
2. When agent completes (notification appears), the frontend freezes
3. Input buffer is cleared - typed text is lost

Expected Behavior

• Frontend should not freeze when background agents complete
• Input buffer should be preserved regardless of background events
• User should be able to continue typing uninterrupted

Actual Behavior

• Frontend freezes briefly when background agent completes
• Any text in the input buffer is lost
• User must retype their message

Environment

• macOS Darwin 24.6.0
• Apple M4, 16GB RAM
• Claude Code (latest as of 2026-01-02)

Impact

High - users lose work (typed text) unexpectedly. In this case, ~100 words of thought were lost.

Suggested Fix

Decouple the background agent completion notification/UI update from the input buffer state. Consider:

• Debouncing UI updates
• Preserving input buffer contents across re-renders
• Using a separate thread/process for notification handling