[BUG] Thai characters display as squares (tofu) in Claude Code CLI - works fine outside Claude

Describe the bug

Thai characters display as squares (tofu/boxes) when using Claude Code CLI. The same terminal displays Thai characters correctly when NOT running inside Claude Code.

Environment

• Claude CLI version: 2.1.12
• Platform: Windows 11 + WSL2 (Ubuntu)
• Terminals tested: Windows Terminal, PuTTY, VS Code integrated terminal
• Locale: C.UTF-8

Steps to reproduce

1. Open any terminal (Windows Terminal, VS Code, PuTTY)
2. Verify Thai characters display correctly: echo "สวัสดีครับ" → Shows Thai text properly
3. Launch Claude Code: claude
4. Claude Code UI and responses show Thai characters as squares (□□□□□)

Expected behavior

Thai characters should display correctly in Claude Code CLI, just like they do in the regular terminal.

Actual behavior

• Thai characters appear as squares/boxes (tofu)
• This happens in ALL terminals when running Claude Code
• Terminal displays Thai correctly when NOT in Claude Code
• Bash command output within Claude Code shows Thai correctly (e.g., echo "ภาษาไทย" works)
• Only Claude Code's own UI/response text renders Thai as squares
• This confirms the issue is within Claude Code's TUI rendering, not the terminal/font

Additional context

This issue affects Thai-speaking users who need to work in their native language. The problem appears to be in Claude Code's TUI (Terminal User Interface) rendering layer, not in the terminal emulator or font configuration.

Related issues

• #3961 - Unicode Input Handling Fails for Vietnamese Characters
• #6094 - Unicode Character Input Corruption
• #1716 - UTF-8 Corruption Bug
• #14686 - Korean characters display issues