Summary

Wrap SessionContextTab and FilesTab in <Show when={sidePanelTab() === ...}> so inactive right-panel tab content fully unmounts instead of staying live.

Two-line change at session-side-panel.tsx:411-413 and session-side-panel.tsx:543-545, matching the existing terminal pattern at line 531.

Why

Area B right-panel audit (#602) found that Context and Files tab content stayed mounted while the user was on another tab, doing real work:

• SessionContextTab recomputes metrics / breakdown / systemPrompt memos and queues a requestAnimationFrame on every message-length change
• FilesTab runs file.load() for each previewable artifact and calls platform.statPaths whenever props.files changes — and props.files itself reacts to Review state, so this fires while users review diffs

This contributes to the typing-lag observation in #615. The other right-panel tabs (Status, Review inner, Terminal) already have explicit guards; Context and Files were the two arms that slipped through.

The fix is not "completely stop all Context-related work" — the tab strip itself, including any per-tab indicators, is unaffected. Only the tab body content unmounts.

Related Issue

Refs #602 (Area B right-panel rewrite)
Contributes to #615 (session UI typing lag)

Human Review Status

Pending

Review Focus

• The Show wrap correctness — does it correctly match each tab id?
• Whether full unmount (vs gating only the heavy effects) is the right choice. Trade-off accepted: Context Accordion expansion state and Files scroll position are lost across remount. Context scroll position is preserved via the existing view().scroll("context") store; Files scroll persistence is not currently implemented and can be added separately if it becomes user-visible.
• Pattern consistency with terminal at line 531.

Risk Notes

• Behavior change: previously inactive Context/Files tabs kept their internal state (open Accordion items, Files scroll position). After this PR, that state resets on tab switch. Accepted as part of the perf win per the audit recommendation.
• No platform, packaging, or shell surface touched.
• No new dependencies.

How To Verify

typecheck:                bun run typecheck — 8/8 packages pass
lint:                     bun run lint packages/app/src/pages/session/session-side-panel.tsx — clean
focused tests:            bun test --preload ./happydom.ts \
                            src/pages/session/session-side-panel.test.tsx \
                            src/pages/session/right-panel-tabs.test.ts \
                            src/pages/session/migrate-session-view.test.ts
                          — 46 pass, 0 fail
visual check (snap):      bun run snap app-shell — passed, screenshot reviewed at
                            docs/design/preview/screenshots/app-shell.png;
                            sidebar + center chat + right-pane rendering unchanged

Manual desktop walk-through of all four tabs (Status / Files / Review / Context) deferred — change is purely additive control flow that mirrors the existing terminal pattern (line 531), and the active-tab render path is unchanged. Reviewer is welcome to request a dev:desktop recording if tab-switching dynamic verification is desired.

Screenshots or Recordings

docs/design/preview/screenshots/app-shell.png regenerated; no visible diff from baseline. Default app-shell snap does not exercise the right panel in an open state, so this confirms no regression to the default surface but does not visually demonstrate the gated tabs.

Checklist

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• I linked the related issue, or stated in Summary why there is no issue.
• I described the review focus and any meaningful risks.
• I replaced the example block in How To Verify with the real verification steps and the key result for each.
• I did not introduce unrelated refactors, dependencies, generated files, or file changes beyond the stated scope.
• (conditional) I manually checked visible UI or copy changes when needed, with screenshots or recordings. Leave unticked only if no visible UI or copy changed.
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