Panic: UTF-8 char boundary error when displaying Korean text in status line

Description

Claude Code crashes with a Rust panic when displaying Korean (한글) characters in the status line or animation output.

Error Message

thread '<unnamed>' (750941) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 2 is not a char boundary; it is inside '어' (bytes 0..3) of `어 × 0.12s + spring)`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort      claude

Root Cause Analysis

The Korean character '어' is a 3-byte UTF-8 character (bytes 0..3). The code attempts to slice the string at byte index 2, which falls inside the multi-byte character boundary, causing the panic.

This appears to occur when the CLI truncates or formats text for display (possibly in status bar or animation output), without properly handling multi-byte UTF-8 characters.

Environment

• Claude Code Version: 2.1.1
• OS: macOS (Darwin 24.6.0)
• Platform: darwin
• Shell: zsh

Steps to Reproduce

1. Use Claude Code CLI with Korean language responses enabled
2. Perform operations that trigger status line animations or text output containing Korean characters
3. The crash occurs when the CLI attempts to truncate or format the Korean text for display

Expected Behavior

The CLI should properly handle UTF-8 multi-byte characters when truncating or formatting text for display, ensuring string operations only occur at valid character boundaries.

Suggested Fix

When truncating strings for display, use character-aware methods instead of byte-based slicing:

• Use str.chars() iterator for character-based operations
• Use str.char_indices() to find valid split points
• Consider using the unicode-width crate for proper display width calculations

Additional Context

The error message 어 × 0.12s + spring) suggests this occurs in animation/timing display code, possibly when showing operation duration or spring animation parameters.