No-flicker mode breaks Korean (CJK) text copy on Windows Terminal

Bug description

When CLAUDE_CODE_NO_FLICKER=1 is enabled, copying Korean (CJK) text from Claude Code output in Windows Terminal produces garbled/mojibake text.

Reproduction steps

1. Set CLAUDE_CODE_NO_FLICKER=1 in environment or ~/.claude/settings.json
2. Launch Claude Code in Windows Terminal (PowerShell)
3. Ask Claude something that produces Korean text output
4. Select and copy the Korean text
5. Paste — the text is garbled

Expected behavior

Korean text should be copied correctly to the clipboard.

Actual behavior

Copied text is garbled with encoding corruption. Example:

Original text (in Claude Code output):

shift 드래그에 대해 얘기하지말것. 터미널스웜 브라우저에선 안되니까. 깨지는건 아래처럼깨져서 나옴.

After copy-paste:

혻shift ?쒕옒洹몄뿉 ????섍린?섏?留먭쾬. ?곕??쏆뒪??釉뚮씪?곗??먯꽌 ?덈릺?덇퉴. 源⑥??붽굔

This is a classic CP949 (Korean legacy encoding) ↔ UTF-8 mismatch pattern, suggesting the alternate screen buffer content is being read with the wrong encoding during clipboard copy.

Environment

• OS: Windows 11 Pro (10.0.26200)
• Terminal: Windows Terminal (latest)
• Shell: PowerShell
• Code page: 65001 (UTF-8, set via chcp 65001 and PowerShell profile [Console]::OutputEncoding = UTF-8)
• Claude Code version: latest
• Setting: CLAUDE_CODE_NO_FLICKER=1

Notes

• English and numeric text copies correctly — only CJK (Korean) characters are affected.
• Disabling no-flicker mode (CLAUDE_CODE_NO_FLICKER=0) resolves the issue, but loses the flicker-free rendering benefit.
• The issue appears to be related to how the alternate screen buffer interacts with Windows Terminal's text selection/clipboard for multi-byte characters.