fix(web): reset markdown widgets when the previewed file changes

[juliusmarminge]

#9677 hoisted the react-markdown component map to a module constant so streaming text reconciles in place instead of remounting the whole message. The Files panel rendered every markdown file through the same unkeyed RenderedMarkdownSurface, so a file switch now reconciled in place too: the previous document's open <details>, wrapped code fences, and pinned table header widths carried into the next file. Before #9677 the component map's memo depended on text, which forced a clean remount on every file change.

Key the rendered markdown surface by relativePath, matching the sibling code-view branches in the same ternary. Updates to the same file (agent edits, workspace refreshes) still reconcile in place, and the chat timeline is unaffected because it keys rows by message id.

Verification

Reproduced in the web app against a two-file fixture project: open README.md rendered, open CHANGELOG.md, expand its <details>, switch back to README.md.

	Before (main)	After
Return to README after expanding CHANGELOG's details

The data-markdown-details-open attribute read true on return with main and false with this change. There is no cheap unit-level regression guard for a key on this branch of FilePreviewPanel (the existing ChatMarkdown tests already cover in-place reconciliation with a stable key), so the proof is the browser check above. tsgo --noEmit in apps/web is clean.

Found by an audit of the perf PRs merged on 2026-09-04. Created with Claude Fable 5 in Claude Code.

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Note

Low Risk
Single React key on the file preview markdown branch; no auth, data, or API changes.

Overview
Fixes stale markdown UI state when switching files in the Files panel rendered preview.

After in-place react-markdown reconciliation (#9677), an unkeyed RenderedMarkdownSurface kept the previous file’s widget state—open <details>, wrapped code fences, pinned table headers—when navigating to another markdown file. This PR adds key={relativePath} on that surface (aligned with other preview branches) so a path change forces a fresh mount while edits to the same file still reconcile in place.

^{Reviewed by Cursor Bugbot for commit 476f8f1. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Reset RenderedMarkdownSurface when previewed file changes in FilePreviewPanel

Adds a key based on relativePath to the RenderedMarkdownSurface instance so React remounts it on file switch. This prevents disclosure and wrap state from the previously displayed file from persisting into the newly selected document.

^{Macroscope summarized 476f8f1.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	+4 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	−2 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.5 KiB	+6 B (+0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	8	8	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.5 KiB	−54 B (−0.4%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.0 KiB	−2 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.5 KiB	−52 B (−0.8%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.9 KiB	57.8 KiB	−88 B (−0.1%)	66.4 KiB	✅
Claude	Live turn messages	10	8	−2 (−20.0%)	21	✅

Baseline: 2fb99a7 · PR result: 476f8f1 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.8 KiB
• Claude decoded thread snapshot: 114.5 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[macroscopeapp]

Approvability

Verdict: Approved at 476f8f1

Macroscope's review found this PR approvable — A path-scoped React key resets markdown-specific UI state when switching files while preserving in-place updates for the currently previewed file. The one-file change is localized to the Files panel and introduces no broader application or configuration impact.

^{You can add or adjust custom eligibility rules. Learn more.}