Korean IME composition renders incorrectly - characters jump during composition

Description

When typing Korean (Hangul) in Claude Code, characters are composed at the bottom of the input area and then jump upward once completed. This makes typing very error-prone and causes frequent typos.

Reproduction

1. Use a Korean IME (macOS default) in Claude Code
2. Type any Korean text
3. Observe that characters shift position during composition

Expected behavior

Characters should compose in-place, as they do in other CLI tools.

Comparison

In the same terminal (cmux), other AI CLI tools handle Korean IME correctly:

• Gemini CLI - characters compose in-place (correct)
• OpenAI Codex CLI - characters compose in-place (correct)
• Claude Code - characters jump during composition (broken)

This confirms it is not a terminal issue but specific to Claude Code input handling.

Environment

• macOS (Darwin 25.3.0)
• Terminal: cmux
• Claude Code version: 2.1.78