[BUG] Response text duplicated in scrollback on ~50% of long responses (VS Code integrated terminal, Windows, v2.1.216) — transcripts confirm rendering-only

Drafted by Claude Code (reviewed by me), including a transcript-level check that narrows this to a rendering bug.

Environment

• Claude Code v2.1.216, native install
• Windows 11 Home 10.0.22621
• VS Code Insiders integrated terminal (Git Bash)
• Model: claude-fable-5

Symptom

Roughly 50% of long assistant responses render with large duplicated chunks in the terminal scrollback — I read a response, keep scrolling, and realize I'm re-reading the same paragraphs again. Whole sections (sometimes what appears to be the entire response) appear twice. It's frequent enough that I now habitually scroll past duplicated blocks.

I have not pinned down a single reliable trigger, but it correlates with long streamed responses in the VS Code integrated terminal (window resizes/panel toggles are common in my workflow, consistent with #51828).

Evidence this is rendering-only (not duplicated API output)

I initially assumed I was paying for duplicated tokens, so I scanned my session transcripts (~/.claude/projects/*/*.jsonl) across my 12 most recent sessions:

• 200 long assistant text blocks checked for a repeated paragraph (>120 chars) within the same message: 0 duplicates
• 207 consecutive long-text message pairs checked for identical/near-identical repeats across messages: 0 duplicates

So the transcript content is clean — the duplication is introduced by the TUI when painting/repainting scrollback, not by the model or API. Good news for billing, but it makes long sessions genuinely hard to read.

Related issues

• #51828 — scrollback duplication on terminal resize, VS Code integrated terminal, macOS (open). This looks like the same bug; filing this as a Windows + v2.1.216 data point with the transcript-side verification above.
• #52773 — Windows terminal duplication report (closed as completed), but the behavior persists here on v2.1.216.

Expected

Each response renders once in scrollback.

Actual

Large chunks of responses appear twice in scrollback in roughly half of long responses; transcript files contain the text only once.