fix(desktop): eliminate mounted-view CPU burn — compositor-safe shimmer, observer append fast path, poll-tick disk reads

[wesbillman]

Summary

Live 0.5.16 review (Royal Court thread) traced the sustained 20–25% WebContent CPU burn in any large mounted channel with a working agent to three defects, in descending impact:

1. Shimmer animation forces per-frame style + compositing walks. .buzz-shimmer animated background-position under -webkit-background-clip: text, which WebKit cannot run on the compositor: every frame did a full document style resolve plus a recursive compositing-hierarchy update over the timeline's layer tree. The highlight now lives on an aria-hidden overlay child duplicating the label text and animates opacity only (compositor-accelerated); a real element is used instead of ::after generated content so screen readers never see the duplicate text. Visual: the moving sweep becomes a gentle pulse. prefers-reduced-motion removes the overlay entirely — static muted label, exactly as before.
2. Observer journal whole-journal dedup + re-sort per append. Every one-per-second observer frame rebuilt a dedup Set over up to 3000 retained events and re-sorted the whole journal with a Date.parse-per-comparison comparator. In-order batches (the ordinary live path, same condition as the existing incremental transcript fold) now dedup within the batch and concat; out-of-order/replayed arrivals keep the full path.
3. Redundant disk reads in the 5s agent-list poll. build_managed_agent_summary re-read the global config from disk per call despite receiving it as a parameter, and re-read the teams store per tracked pair — 2N redundant reads per poll tick for N agents. Both are now caller-supplied; one-shot command paths use a summarize_from_disk helper. Same stores, same unwrap_or_default failure posture, read once.

Evidence (mechanism attribution, live 0.5.16-block, mounted ~1500-event channel)

• True-idle mounted view (working-state UI live): 25.08% mean / 24.15% median WebContent CPU; post working-state decay: 6.33% / 3.50%.
• Reduce Motion A/B during a live agent turn (isolates the shimmer, working UI still mounted): 19.50% mean → 5.39% mean (72% collapse). Native 10s samples: Document::resolveStyle 379 samples → 1; updateCompositingLayersAfterStyleChange 378 → 0; recursive updateBackingAndHierarchy 363 → 6.
• The shimmer mechanism predates 0.5.16 (CSS unchanged since Polish composer activity layout and transitions #3151); current multi-agent workloads exposed and amplified it. The mention regression itself was fixed separately in fix(desktop): restore release agent mentions #6182.

Testing

• desktop observer store suites: 55/55 pass, including 5 new tests pinning the fast-path invariants (equal-timestamp seq ordering, duplicate-batch redelivery, intra-batch duplicates, overlapping late arrival takes the slow path, transcript-equals-replay on both paths).
• cargo test --lib managed_agents (1016 passed) and --lib commands (727 passed); cargo clippy clean; pnpm typecheck + biome clean.
• Release gate for the patched build (per Mongo): active-turn mounted CPU must collapse from the ~21–25% baseline with no per-frame style/compositing walk in a native sample — measured on Wes's workspace once a build with this branch is running.

Findings and raw samples: RESEARCH/LIVE_0_5_16_WEBKIT_ATTRIBUTION_2026_08_18.md, RESEARCH/RENDERER_STEADY_STATE_LOOP_AUDIT_2026_08_18.md, .scratch/live-app-review/ (Carl/Donut/Mongo/Brain, Royal Court thread 01a7fe75).

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Opened by Brain (agent) via @wesbillman's account on his behalf — coordinated in Buzz channel agent-mention-policy-royal-court, thread 01a7fe75.