Edit tool diff preview renders entire file as all-green additions (no removals, no context)

Symptom

When Claude Code proposes an Edit tool change, the diff preview shows the entire new file content as one big green block — no red removed lines, no -/+ interleaving, no context lines. Just the full new content rendered as all-additions. Happens for tiny single-line edits to a normal-sized Python file (~200 lines).

Reproduction

1. In a Claude Code session, ask Claude to make a one-character or one-line change to an existing Python file.
2. Observe the Edit tool's diff preview before approving.

Expected: unified diff with red - removed lines and green + added lines, with surrounding context.

Actual: full new file content shown in green, old lines absent entirely.

Environment

• Claude Code version: 2.1.141
• OS: WSL2 Ubuntu on Windows 11
• Node: v24.15.0 (bundled binary)
• Settings: {"model": "opus", "theme": "auto"} — no other overrides
• TERM: xterm-256color
• COLORTERM: unset
• CLAUDE_EFFORT: xhigh

Ruled out

• Not line endings — file is LF, file confirms no CRLF.
• Not the JetBrains plugin's IDE diff handoff — persists with ENABLE_IDE_INTEGRATION unset and CLAUDE_CODE_SSE_PORT unset.
• Not the PyCharm "Reworked Terminal" — switched to Classic Terminal, reproduces. Also reproduces in standalone Ubuntu/WSL terminal outside PyCharm entirely.
• Not a per-project setting — project has no .claude/settings.json or CLAUDE.md.

Notes

Binary string search shows the renderer has DiffTooLarge, Binary file - cannot display diff, diffAdded/diffAddedDimmed/diffRemoved/diffRemovedDimmed constants, so a fallback code path appears to exist — possibly being triggered incorrectly here.

Happy to provide more diagnostic info (debug logs, etc.) if useful.