perf: defer off-screen transcript render + fix dropped Ctrl+Down keystroke#584
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…trl+↓ keystroke Two user-facing perf wins from goal/profile-si-320532b0, cherry-picked clean onto master without the Phase 1 instrumentation harness. Opt-A — defer off-screen transcript render (default-ON behind perf flag) ======================================================================== `<message-list>` now wraps each off-screen transcript item in a `<deferred-block>` placeholder. The bottom 8 items render eagerly (the user is scroll-to-bottom on session activation, so the tail is what first paint shows). Older items render a cheap height-preserving div; an IntersectionObserver (rootMargin 500px) resolves them on approach via requestIdleCallback. A Ctrl+F / Cmd+F / F3 escape hatch force-resolves every placeholder so native browser-find sees the full transcript. A/B on the realistic-large fixture (n=5): paint.first p95 -37ms (-33%) nav.session.ready p95 -95ms (-29%) rapidnav.keystroke p95 -102ms (-32%) rapidnav.gap p50 -67ms Disable via `localStorage.bobbitPerfFlags=-deferOffscreenRender`. Opt-E — fix dropped Ctrl+↓ keystroke during slow attach ======================================================== `getActiveNavId` previously gated the keyboard-nav override on `window.location.hash === expected`. Session navigation is async (dynamic import + connectToSession), so a rapid second keystroke would fire before the first nav's setHashRoute had committed, and re-derive active id from the stale URL — looping back to the same row and dropping the keystroke. The override is now trusted unconditionally; the existing hashchange listener clears it on stale URLs. Co-authored-by: bobbit-ai <bobbit@bobbit.ai>
…ferOffscreenRender docs Co-authored-by: bobbit-ai <bobbit@bobbit.ai>
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Summary
Two surgical, user-facing changes uncovered by a perf investigation (full instrumentation + harness + cross-commit report are parked on `goal/profile-si-320532b0` as a follow-up):
Opt-A — defer off-screen transcript render (perf win)
Sidebar navigation into long sessions was synchronously rendering the entire transcript before first paint, even though only ~3 messages are in viewport after auto-scroll-to-bottom. Now: the last 8 messages render eagerly; older messages render a height-preserving placeholder and swap in via `IntersectionObserver` + `requestIdleCallback` as the user scrolls. Native `Ctrl+F` / `Cmd+F` / `F3` is captured at the keydown level to force-resolve all placeholders before the browser's find UI runs.
Behind a perf flag `deferOffscreenRender` (default-ON). Set `localStorage.bobbitPerfFlags=-deferOffscreenRender` to disable.
Measured impact on a realistic-large transcript fixture (n=5 replicates per arm, every ON sample beats every OFF sample on the headline spans — ranges don't overlap):
User-facing meaning: rapid Ctrl+↓ walks through the sidebar now feel smooth — the main thread frees up between keystrokes well within the 100 ms snappy budget.
Opt-E — dropped Ctrl+↓ keystroke fix (correctness)
Bug: `getActiveNavId()` in `src/app/sidebar-nav.ts` discarded `state.keyboardNavActiveId` when the URL hadn't yet caught up to the override. During a ~200 ms session attach, 3–4 of 6 rapid Ctrl+↓ presses were re-routed to the same row.
Fix: trust the override unconditionally — the existing hashchange listener already clears it on stale URLs. Single-site change, pinned by `tests/rapid-keystroke-nav.spec.ts`.
Files
11 files, +946 / -7. Strictly the two features + their pinning tests + a minimal perf-flags helper. No server changes, no docs, no instrumentation overhead, no harness.
Verification
Follow-up
The branch `goal/profile-si-320532b0` (parked, PR #579) contains the supporting infrastructure that enabled this win: perf-trace primitive, manual harness, cross-commit report, real-session JSONL profile, `HOW-TO-REPEAT.md`. Worth landing later as a separate PR for future perf work; not in scope here.
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