[BUG] Ctrl+J external editor: user input block overlays assistant output on scroll (Windows)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

After submitting multi-line input via Ctrl+J (external editor), the user input block's background color overlays/covers the assistant's response when scrolling up. The assistant output underneath becomes unreadable — the user input block visually "paints over" it.

This does not occur with regular input (typing directly in the prompt). Tested with equivalent volume of text via normal input — assistant output renders and scrolls correctly.

Steps to Reproduce

1. Start a Claude Code session on Windows
2. Have an ongoing conversation with some assistant output visible
3. Press Ctrl+J to open external editor
4. Type or paste a large multi-line message (10+ lines)
5. Save and close the editor to submit
6. Wait for the assistant's response
7. Scroll up — the user input block's background covers the assistant output above it

Expected Behavior

User input block and assistant output should occupy separate visual regions. Scrolling should not cause one block to overlay another.

Actual Behavior

The user input block's rendered area (dark background) extends over the assistant's response area when scrolling, making the assistant output unreadable. The overlap worsens with longer external editor inputs.

Related Issues

• #32467 — Similar external editor TUI repaint issue (output disappears after Ctrl+G, macOS)
• #21267 — Historical Windows text rendering overlap (closed, v2.1.19)

Environment Info

• Claude Code: 2.1.89
• Platform: win32 (Windows 11 Build 26200)
• Shell: PowerShell 7.6.0 (PSEdition: Core)
• Terminal: Windows Terminal
• Node.js: v24.11.1
• TUI shell (internal): bash (git-bash) with TERM=xterm-256color