fix(app): anchor virtual timeline to bottom

[Hona]

Summary

• Anchor the virtual timeline to the last row on initial mount when we are in normal latest/follow-bottom mode.
• Keep hash/deep-link/pending-message/user-scrolled paths out of the anchor path.
• Use virtua's itemSize hint only when no exact row-key cache exists, so cold loads avoid the default ~40px estimate.

Measurement setup

• Base: latest origin/perf/session-timeline-virtua at 5c0d9ed030 after cache PR merge.
• URL/origin: http://127.0.0.1:3002.
• Target static session: fix: Flicker on load / ses_1e62fb38dffegWEV7SJWQKxJXZ.
• Review panel closed/unmounted: reviewCounts: [0, 0].
• Heavy settings verified on both runs: reasoning summaries, shell parts expanded, edit parts expanded.
• Measurement source: Chrome DevTools rAF DOM sampling + MutationObserver + Chrome performance traces.

Revisit with cache: baseline vs PR

This measures the first candidate change. The PR path uses its own generated cache snapshot, so total scrollHeight is not treated as an apples-to-apples truth metric here; the useful signal is whether a rendered frame exposes the temporary top/zero-scroll state.

Metric	Baseline cache branch	This PR
First sampled root	47ms	111ms
First sampled root scrollTop	0	17239
First sampled root bottom-aligned	no	yes
rAF samples with session root at scrollTop=0	1	0
First bottom-aligned sample	88ms	111ms
Height-change samples	12126 only	18237 only
Trace	virtua-followup-static-baseline-trace.json.json.gz	virtua-followup-static-anchor-trace.json.json.gz

Cold load, no cache: baseline vs PR

This measures the second candidate change (itemSize fallback) plus the anchor.

Metric	Baseline cache branch	This PR
Initial total estimate	2600px	3900px
First sampled root scrollTop	0	2902
rAF samples with session root at scrollTop=0	1	0
Height correction sequence	2600 -> 9969 -> 11488 -> 12126	3900 -> 8444
First bottom-aligned sample	807ms	113ms
Trace	virtua-followup-cold-baseline-trace.json.json.gz	virtua-followup-cold-final-trace.json.json.gz

Validation

• packages/app: bun typecheck
• push hook: bun turbo typecheck

Notes

• itemSize=60 is an official virtua size hint and is only active without a matching cache. It is still an estimate by design; exact row-key cache remains preferred when available.
• I did not add min-height reservations or custom measurement logic.

[Copilot]

Pull request overview

This PR updates the session message timeline virtualization behavior to avoid an initial “top/zero-scroll” flicker by anchoring the virtualized list to the bottom on initial mount (only in normal follow-bottom scenarios), and improves cold-load estimation by supplying a better virtua itemSize hint when no row-key cache is available.

Changes:

• Add a bottom-anchor path in MessageTimeline that scrolls to the last row once per session key when shouldAnchorBottom() allows it.
• Thread a shouldAnchorBottom() predicate from session.tsx to keep deep-link/hash/pending-message/user-scrolled cases out of the anchor path.
• Provide virtua an itemSize fallback (60px) only when no virtualizer cache exists.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/app/src/pages/session/message-timeline.tsx	Adds one-time bottom anchoring logic per session and uses an itemSize hint only when cache is absent.
packages/app/src/pages/session.tsx	Supplies shouldAnchorBottom() based on URL hash, deep-link state, pending message, and user-scrolled state; adds useLocation() usage.

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