[teamleaderleo]

Leo (rulebook): "when it comes to the left scroll rail thing, i already said we want it ultra fast ultra responsive". Leo, 2026-10-29: "giving it faster scrttoll/hover over ... and being able to scroll it".

Stacked on #28494 (head ca1f3e2) until it lands. Until then the diff against base includes #28494's commits. Rebased onto #28494's merge with #24131 (the right-edge rail): the card places itself on either edge (left: card.left == rail.right; right: card.right == rail.left, in the layer by its own width or pinned to the right edge without one).

Takes back #26134: the card now measures and places itself in one commit (fit() plus a ResizeObserver in PreviewCard) instead of a setCardHeight round-trip; #26134's wrapped-prompt behavior is kept and minimap-card-layout.test.ts still passes.

Root cause: why the rail harness cases timed out at 3,000 prompts

The cause was the harness, not the rail. Three harness faults hit both arms:

1. The preview server colours its port in its startup banner, so the harness never saw it start and then hung until the 1800 s CI limit. Fixed in Session rail: the card opens on hover, a click lands at once, and wheeling restyles no tick #28494.
2. A hover never started the probe's clock, because the probe's default inputs are presses, keys and wheels. The hover cases now arm on pointerover.
3. The flows fixture opens Session Info, which hid the rail on base. The cases now close Session Info first.

With the harness fixed, the rail does no work that grows with the number of prompts. The 3,000 ticks are already virtualized to about 86 drawn. A Chromium trace at 300 and at 3,000 prompts gave the same per-step cost: wheel style time 233 vs 239 ms per 10 steps, and hover 19–36 ms to commit. The remaining cost is per pane DOM, not per prompt:

• Hover (fixed here): opening the card took 3 React commits and 6 forced layouts, because the card's origin, its height and its reply line budget each went through state.
• Wheel (not fixed here): any node inserted under the agent pane restyles the whole pane in Chromium, about 1,100–1,165 elements at 4.6 ms. Six pane :has() rules cause this (the composer field menu, the PR dock's :has(> .acpmux-pr-dock) > :not(...), and two label:has(...) rules). With every :has() rule turned off, the same wheel step restyles 14–22 elements. A wheel step over the rail inserts nodes in two places: ticks entering the drawn window, and the open card's reply when the hovered row changes. Those rules are being handled in a separate PR, since they also affect transcript streaming.
• I also tried snapping the tick window to 32-row chunks, so the mounted ticks change less often. It made no measurable difference (wheel p95 31.4 vs 30.5 ms on base, 12 vs 12 dropped frames), so it is not in this PR.

Change

• The preview card now positions itself in its layout effect. It reads the rail's origin and its own height and writes its transform before paint. It writes the reply line budget straight onto the reply. Its owner passes the row's center and the view height, not a top it had to measure for. The card now opens in one commit. The existing ResizeObserver keeps the card placed when its content grows; it is disconnected when the content changes or the card goes. This adds no timer or observer, and no work per prompt.
• The harness gains four cases at 3,000 prompts: chat.rail.hover.tick, chat.rail.hover.card, chat.rail.wheel and chat.rail.click.

Performance

Same harness and same command for every arm: bun scripts/latency/run.ts --engine chromium --runs N --page "chat rail pointer", on the 3,000-prompt resident flows page, Chromium. Within a run, the arms differ only in webviews/src: base is 094dd560 (feat-cmux-next before #28494), "#28494" is f176bb2, "this PR" is the card change on top of it. The card change was measured before the rebase onto #24131; the rebase adds only the right-edge branch of its placement.

metric	before	after	delta	harness or run link
hover → card painted p50 / p95, vs feat-cmux-next	289.6 / 292.8 ms	41.3 / 42.7 ms	−248 ms (−86%); the 250 ms dwell is #28494's, so −3 ms here	latency chat.rail.hover.card, runs=10, run 41136916156
hover → card painted p50 / p95, vs #28494	44.1 / 46.0 ms	41.3 / 42.7 ms	−2.8 ms (−6%)	same case; #28494 run 41139037067, this PR run 41136916156
React commits, 6 hover cases, vs #28494	18	7	−61%	chat.rail.hover.tick, runs=6, run 41139037067
hover → tick lit p50 / p95, vs #28494	45.3 / 46.5 ms	43.6 / 45.6 ms	−1.7 ms	same run
wheel frame time p50 / p95 / dropped (10 × 100 px, 6 runs), vs feat-cmux-next	16.7 / 30.5 ms / 12	16.7 / 28.8 ms / 14	within noise	chat.rail.wheel, base run 41138308077, this PR run 41139037067
wheel React commits (6 runs), vs feat-cmux-next	120	60	−50%	same runs
click → prompt landed p50 / p95	84.1 / 115.9 ms	93.4 / 127.2 ms	within run-to-run noise (#28494's head arm: 83.6 / 115.2)	chat.rail.click, runs 41138308077 / 41139037067

Harness note: when the cases share one page, a case that runs after another sometimes finds no rail ("rail not ready ... rail:false"), so some arms lack a case in a given run. The table uses only complete cases and links each to its run. The wheel and click costs are the pane :has() restyle described above, which is being fixed separately.

Stills

The card's placement code changed, so here are gallery stills with the hover card open on both edges. They were captured via cmux-ci in one job (run 41144770735), Chromium. Before is #28494's head ca1f3e26; after is this PR's head 444a364e. Left edge: agent-pane.thread-minimap / card-markdown. Right edge (#24131): agent-pane.thread-minimap-narrow / right-hovered. All eight before/after pairs are byte-identical (same PNG hash): the card lands on the same pixels on both edges, with card.left == rail.right on the left and card.right == rail.left on the right.

	Before	After
Left, dark
Left, light
Right, dark
Right, light

Viewed after image: yes

Testing

• Rail jsdom suites (*minimap*, hover-intent-cards, minimap-*, thread-scroll-map*, HarnessStatusCard.dom): 134 pass, 0 fail.
• Card browser suites after the rebase (thread-minimap-feel, thread-minimap-motion, thread-minimap-side (right edge), thread-minimap-left-rail-stays, minimap-card-layout, thread-minimap-card-glass, thread-minimap-tick-states), Chromium and WebKit: 54 pass, 0 fail (run 41142877669).
• scripts/web-lint.sh: passes.

Changelog

Changed: The chat rail's preview card opens with less work per hover.

Agent session: agent session

🤖 Generated with Claude Code

[blacksmith-sh]

Found 6 test failures on Blacksmith runners:

Failures

Test	View Logs
main.DeveloperIdAppTests.test_download_prints_the_active_profile_only/ test_download_prints_the_active_profile_only	View Logs
stubs: /System/Library/Frameworks/Security.framework/Versions/A/Security has no stub/ Security has no stub	View Logs
stubs: CoreFoundation.tbd: install-name /System/Library/Frameworks/ CoreFoundation.framework/Versions/A/CoreFoundation, missing 1: ['_CFRetain']/ CoreFoundation, missing 1: ['_CFRetain']	View Logs
stubs: CoreFoundation.tbd: install-name /System/Library/Frameworks/ CoreFoundation.framework/Versions/A/CoreFoundation, missing 1: ['_log2']/ CoreFoundation, missing 1: ['_log2']	View Logs
test_every_listed_hook_is_one_glaeda_released_with_the_check (main.SigningHooksInSt epWithGlaedaTests.test_every_listed_hook_is_one_glaeda_released_with_the_check)/ test_every_listed_hook_is_one_glaeda_released_with_the_check (main.SigningHooksInSt epWithGlaedaTests.test_every_listed_hook_is_one_glaeda_released_with_the_check)	View Logs
test_the_ten_first_party_gpl_crates_are_named_exactly (main.AppCrateLicenseGate.tes t_the_ten_first_party_gpl_crates_are_named_exactly)/ test_the_ten_first_party_gpl_crates_are_named_exactly (main.AppCrateLicenseGate.tes t_the_ten_first_party_gpl_crates_are_named_exactly)	View Logs

^{Need help on this PR? Tag @codesmith-bot with what you need.}

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[github-actions]

cmux status _{(one comment, edited in place; each line is updated by the job that owns it)}

Review e09e164: HQ feed card · gallery before/after runs · checks

App of this head (app.zip, kept 1 day)

gh workflow run reload-build.yml -R manaflow-ai/cmux --ref feat-cmux-next -f tag=pr28540 -f ref=lane/ux-rail-3000 -f platform=macos -f backend_mode=local

Stale: these images are of 444a364; the head is now e09e164 and a new capture is running.

Gallery before/after for 444a364: 16 states rendered, 20 images differ, 1337 more touched states not rendered. Images are in the run's artifact, not on this public PR.

gh run download 41144144432 -R manaflow-ai/cmux -n gallery-advisory-28540-444a364e838d634cc909015d4da6d8bf6d07553d (then read gallery-review.json)

States

State	Theme	Status	Diff	Picked by
agent-pane.thread-minimap/card-markdown	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-markdown	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-failure	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-failure	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-metadata	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-metadata	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-prompt-fixed-2	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-prompt-fixed-2	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-prompt-fixed-3	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-prompt-fixed-3	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-prompt-growing	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/card-prompt-growing	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/rest	Apple System Colors Light	unchanged	0%	own PreviewCard.tsx
agent-pane.thread-minimap/rest	Apple System Colors	unchanged	0%	own PreviewCard.tsx
agent-pane.thread-minimap/scrubbing	Apple System Colors Light	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap/scrubbing	Apple System Colors	changed	0%	own PreviewCard.tsx
agent-pane.thread-minimap-height/eighteen-prompts	Apple System Colors Light	changed	1.67%	own ThreadMinimap.tsx
agent-pane.thread-minimap-height/eighteen-prompts	Apple System Colors	changed	0.678%	own ThreadMinimap.tsx
agent-pane.thread-minimap-height/seventy-prompts	Apple System Colors Light	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-height/seventy-prompts	Apple System Colors	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-height/tall-pane-caps	Apple System Colors Light	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-height/tall-pane-caps	Apple System Colors	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-lit/mid-thread	Apple System Colors Light	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-lit/mid-thread	Apple System Colors	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-lit/at-the-end	Apple System Colors Light	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-lit/at-the-end	Apple System Colors	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-lit/hovered	Apple System Colors Light	changed	0.869%	own ThreadMinimap.tsx
agent-pane.thread-minimap-lit/hovered	Apple System Colors	changed	0.67%	own ThreadMinimap.tsx
agent-pane.thread-minimap-mark/states	Apple System Colors Light	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-mark/states	Apple System Colors	unchanged	0%	own ThreadMinimap.tsx
agent-pane.thread-minimap-mark/bookmark-hovered	Apple System Colors Light	changed	2.51%	own ThreadMinimap.tsx
agent-pane.thread-minimap-mark/bookmark-hovered	Apple System Colors	changed	1.13%	own ThreadMinimap.tsx

_{Pick other states with a Gallery: entry.id, entry.id/variant line in the PR body.}

Shipped in:

• cmux NEXT nightly 1.0.0-nightly.4115755828101 (7c0adaf184)
• cmux NEXT nightly 1.0.0-nightly.4115843934401 (8b3fa55ef5)
• cmux NEXT nightly 1.0.0-nightly.4115962701801 (631d310eb9)

[teamleaderleo]

Review (codex gpt-6-astra high, router-review): BLOCK at e09e164

BLOCK

Visual verdict: PASS — all four after screenshots were inspected; card placement looks correct on both edges and themes, and hashes match the before images.

1. webviews/src/agent-session/acpmux/threadMinimap/PreviewCard.tsx:174-179 — Reply-only cards (used by ScrollMap for assistant marks) return after removing the prompt card’s inline reply clamp, without calling place() or installing a ResizeObserver. When hover swaps from a prompt card to a reply card, the component is reused: placement measures the stale clamped height, then cleanup changes the height while leaving the old transform. The card can therefore be vertically misplaced, and later width/content-driven size changes are not repositioned.

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[teamleaderleo]

Review (codex gpt-6-astra, router-review): SHIP at 1f32556

Delta review from e09e164 (prompt and reply cards share one fit() path; ResizeObserver cleanup). No findings. Visual verdict not part of this range review.