fix(app): make session navigation stable and fast

[Hona]

Summary

• keep the previous coherent session painted until destination placement is ready, without adding a loading screen
• reuse bounded server/session placement and small opaque cursor pages to reduce cold navigation work
• preload likely route modules, every open session tab, and the top home sessions with owned cleanup
• prepare completed Markdown during the existing bounded home prefetch and stage review internals after session content
• make titlebar tabs use authoritative cached session metadata before issuing another request
• make one history action consume one opaque cursor page while preserving visible anchors and bounded continuation
• add discoverable painted-frame benchmarks for cold/hot tabs, child sessions, new sessions, home-to-session content/tab paint, and single-tab close-to-home

Performance

Production Chromium, native speed, one worker. Painted-frame benchmarks sample with requestAnimationFrame followed by setTimeout(0). Baseline is current upstream/dev at e8610d821; branch is the PR head. First-navigation and home/close values are medians of 3 runs. Cold/hot tab values are medians of 15 trials (3 benchmark repetitions, each containing 5 trials).

Windows

Scenario	Metric	Before	After	Change
Cold session	First correct paint	92.9 ms	20.7 ms	78% faster
Cold session	Stable paint	108.4 ms	44.6 ms	59% faster
Child/subagent	First correct paint	57.5 ms	39.3 ms	32% faster
Child/subagent	Stable paint	72.4 ms	60.7 ms	16% faster
New session	First correct paint	37.0 ms	38.1 ms	3% slower
New session	Stable paint	65.3 ms	61.5 ms	6% faster
Cold tab	First correct paint	67.9 ms	23.2 ms	66% faster
Cold tab	Stable paint	134.1 ms	45.3 ms	66% faster
Hot tab	First correct paint	24.0 ms	24.1 ms	unchanged
Hot tab	Stable paint	46.6 ms	46.4 ms	unchanged

macOS

Scenario	Metric	Before	After	Change
Cold session	First correct paint	58.6 ms	38.9 ms	34% faster
Cold session	Stable paint	80.1 ms	46.7 ms	42% faster
Child/subagent	First correct paint	56.9 ms	54.7 ms	4% faster
Child/subagent	Stable paint	80.6 ms	63.8 ms	21% faster
New session	First correct paint	48.3 ms	48.4 ms	unchanged
New session	Stable paint	80.0 ms	80.4 ms	unchanged
Cold tab	First correct paint	58.3 ms	41.2 ms	29% faster
Cold tab	Stable paint	97.4 ms	47.3 ms	51% faster
Hot tab	First correct paint	16.3 ms	16.4 ms	unchanged
Hot tab	Stable paint	47.7 ms	47.7 ms	unchanged

Home And Tab Milestones

Home-to-session records session content and titlebar-tab visibility independently. Closing measures home shell, restored session row, and tab removal.

Platform	Scenario	Metric	Before	After	Change
Windows	Home → session	Session content first paint	166.8 ms	79.8 ms	52% faster
Windows	Home → session	Titlebar tab first paint	166.8 ms	10.9 ms	93% faster
Windows	Home → session	Content + tab stable	190.1 ms	97.5 ms	49% faster
Windows	Close only tab → home	Home + row + tab removal first paint	81.0 ms	48.5 ms	40% faster
Windows	Close only tab → home	Stable paint	111.3 ms	78.5 ms	29% faster
macOS	Home → session	Session content first paint	110.4 ms	68.7 ms	38% faster
macOS	Home → session	Titlebar tab first paint	110.4 ms	36.0 ms	67% faster
macOS	Home → session	Content + tab stable	137.5 ms	82.1 ms	40% faster
macOS	Close only tab → home	Home + row + tab removal first paint	55.1 ms	54.5 ms	unchanged
macOS	Close only tab → home	Stable paint	80.5 ms	64.0 ms	20% faster

Visual Stability

Metric	Before	After
Cold session blank frames	1-3	0
Child/subagent blank frames	2	0
Cached tab blank/wrong frames	0	0
Streaming bottom drift	0	0
Timeline/Markdown remounts	0	0

Chrome Trace Attribution

Full Chrome Performance traces include V8 sampling and timeline-stack overhead, so these values are reported separately from the native-speed medians above. User Timing marks delimit click, first paint, and stable paint inside each trace.

Platform	Trace metric	Previous branch	Latest branch	Change
Windows	Home → content first paint	163.1 ms	104.7 ms	36% faster
Windows	Long synchronous mount task	135.8 ms	85.9 ms	37% shorter
macOS	Home → content first paint	95.6 ms	81.4 ms	15% faster
macOS	Long synchronous mount task	53.9 ms	42.1 ms	22% shorter

The remaining traced cost is dominated by final Solid/DOM construction and layout rather than network or Markdown parsing. Review-shell geometry remains immediate; only its internal body is staged after the first session-content paint.

Verification

• bun turbo typecheck across 23 packages
• bun run test:unit: 415 passed
• UI tests: 58 passed
• bun test ./e2e/performance/unit: 15 passed
• bun run test:e2e: 16 passed
• timeline smoke, delayed cursor prepend, all-open-tab prefetch, collapse state, context resize, prompt hydration, Markdown cache, and virtualizer browser tests
• 320-turn, 160-delta streaming stress: every delta delivered, zero blank samples, zero bottom drift, zero timeline/Markdown replacement
• repeated production benchmarks on macOS and Windows against exact current upstream/dev and rebased branch refs

Notes

• no loading page, splash, CSS, copy, or intentional visual design change
• the existing message API remains in use; pagination is tested with opaque cursor tokens
• bun run test:bench discovers all navigation scenarios above for future baseline comparisons

[Copilot]

Pull request overview

This PR focuses on improving session navigation paint stability and latency in the app by caching session “placement” (server + directory + root) to avoid cold route work, switching history pagination to single opaque-cursor pages, and adding performance benchmarks to guard the new behavior.

Changes:

• Add a bounded session placement cache and wire it into home/session navigation and titlebar tab metadata to reduce cold navigations.
• Adjust timeline history paging to consume exactly one opaque cursor page per action, with controlled continuation when the user is already at the top.
• Add/expand e2e performance probes + fixtures to measure first/stable paints for navigation milestones and first navigation blank-frame behavior.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/app/src/utils/session-placement.ts	New bounded placement store used to resolve directory/root quickly across navigations.
packages/app/src/utils/session-placement.test.ts	Unit tests for placement aliasing, inheritance, and bounding behavior.
packages/app/src/pages/session/timeline/model.ts	Simplifies “load older” to a single page per call (opaque cursor semantics).
packages/app/src/pages/session/timeline/model.test.ts	Updates tests to reflect single-page history loading behavior.
packages/app/src/pages/session.tsx	Adds request/continuation coordination for history paging and cleanup for scheduled frames.
packages/app/src/pages/session-module.ts	Adds shared dynamic import wrapper for session module preloading.
packages/app/src/pages/home.tsx	Preloads session module; prefetches top sessions; sets placement before navigation; adds tabs usage.
packages/app/src/pages/home-module.ts	Adds shared dynamic import wrapper for home module preloading.
packages/app/src/pages/directory-layout.tsx	Inherits placement for child-session navigation to keep destination placement ready.
packages/app/src/context/global.tsx	Registers the global sessionPlacement store alongside existing global contexts.
packages/app/src/context/directory-sync.ts	Reduces initial message page size to support small-page navigation/pagination strategy.
packages/app/src/components/titlebar.tsx	Uses cached placement + local sync data for tab titles before falling back to network; preloads home module.
packages/app/src/app.tsx	Switches session-route resolution from query placeholdering to resource + placement cache.
packages/app/e2e/utils/mock-server.ts	Introduces opaque cursor tokens for message pagination in e2e mock server.
packages/app/e2e/smoke/session-timeline.spec.ts	Makes timeline key sampling robust by collecting currently visible timeline part IDs.
packages/app/e2e/performance/unit/timeline-test-helpers.test.ts	Adds unit test for stress-session link builder.
packages/app/e2e/performance/unit/navigation-milestones.test.ts	Adds unit test for milestone first/stable paint summarization.
packages/app/e2e/performance/unit/first-navigation-metrics.test.ts	Adds unit tests for first-navigation blank-frame/stability summarization.
packages/app/e2e/performance/timeline/timeline-test-helpers.ts	Updates stress helpers for server-scoped routes, draft tabs, and fixture paging.
packages/app/e2e/performance/timeline/session-timeline-stress.fixture.ts	Extends fixture with child session + tool metadata to exercise child navigation.
packages/app/e2e/performance/timeline/navigation-milestones.ts	Adds probe + summarizer for content/tab/home/tab-removed milestone paints.
packages/app/e2e/performance/timeline/home-tab-navigation-benchmark.spec.ts	Adds benchmarks for home→session and close-only-tab→home milestone paints.
packages/app/e2e/performance/timeline/first-navigation-probe.ts	Adds probe for detecting blank frames during first navigation to destination.
packages/app/e2e/performance/timeline/first-navigation-metrics.ts	Adds summarizer for first navigation samples (blank/source/destination).
packages/app/e2e/performance/timeline/first-navigation-benchmark.spec.ts	Adds benchmarks for unvisited tab, new-session draft, and child session first-navigation paint.
packages/app/e2e/performance/README.md	Documents additional benchmark scenarios covered by the suite.

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