[teamleaderleo]

Leo-ask: ask-a5c1976d1c
Leo-ask: ask-d974edac49
Leo-ask: ask-c07aebfca2
Leo-ask: ask-0bfebc52fd

Leo: "and we shuold also have some animation for collapsing and expanding" (the agent chat's transcript folds), and: "these situations when collapsibles get expanded i said earlier last night lawrence was talking about how it shouldnt move your scroll or whatever just expand the area at the bottom and you're in the same place... also this current one seems to shift the expandable area away also these all need animations for expand and collpase!", and: "and thne there were a few animations that didnt quite work for collapsing and expanding wrt tool calls and stufff".

What #23093 already covers (merged 2026-10-21): tool groups ("Ran 4 commands ›"), tool, command and MCP rows, settled runs and thoughts open and close with the shared fold (fold.ts). The body's height and opacity animate over --cv-fold-duration (the host's move token, about 180 ms; 0 under Reduce Motion or with cmux animations off), the chevron turns on the same token, and a click on a button.is-toggle holds that line in place.

Scope: motion and anchoring only. This PR doesn't decide what goes inside a "Worked for" fold. Leo's takeaway rule lives in the shared turns.ts function that tp/fold-interim-messages owns (#24300). This PR doesn't touch turns.ts.

Split to stay under 800 lines. This PR is the shared primitive and every in-row collapsible. The "Worked for" and subagent-group row motion (whole virtual rows sliding and fading, turnFold.ts) follows in its own PR once this one lands.

What this PR does

1. One shared collapsible, useAnchoredCollapse (conversation/useAnchoredCollapse.ts): useAnchoredCollapse({ defaultOpen?, open?, onOpenChange?, animates? }) -> { open, present, toggle, setOpen, headerRef, bodyProps }.
   • The header stays on its pixel and the body grows or shrinks below it, on the shared fold.
   • Inside the transcript, the header sends a bubbling cmux:anchored-collapse event that VirtualTranscript answers with the same hold cmux NEXT: expanding a tool call or thought animates and never moves your view #23093's click path uses. Keyboard toggles, and headers that are not button.is-toggle, are held too.
   • Outside the transcript, the hook corrects the nearest scroller itself on each frame until the fold ends.
   • Tool groups, tool, command and MCP rows, background task notes, thoughts and the one-file edited-files diff all go through it. agent-message-style adopts it for the agent-message card in its own PR.
2. Show more / Show less on a long prompt now animate.
   • Show more holds the bubble's top, so the text grows below it, and the button rides the bubble's bottom edge instead of jumping.
   • Show less holds the button where it was clicked, and the text shrinks back above it.
   • A reversal midway starts from the drawn height, and unmounting cancels what plays.
3. The audit fixes (frame strips below, every collapsible in Chromium and WebKit):
   • The one-file edited-files diff appeared and vanished in one frame. It now goes through the hook.
   • A fold whose body draws its content late (the diff, highlighted code) opened to the few pixels drawn at the click, then snapped to full height. useFold now follows the content as it grows: each larger size takes over from the drawn height for the time left. Its ResizeObserver stops when the fold ends, when another starts, or on unmount.
   • Show more and Show less had no animation at all.
   • Show less on a prompt several screens tall, with motion off, lost its hold. The row shrank under the old offset and the virtualizer unmounted it, taking focus with it. While a hold lasts, the row it holds stays in the mounted ranges, and when the hold moves the offset, the mounted rows take the new offset in the same frame.
   • Tool rows, command groups, MCP calls, thoughts, background task notes and agent-message cards: each header stayed within 0 px in both engines, with no element running two animations at once (table below).

Works with tp/fold-no-blank-space: this PR doesn't touch reportDrawn, placeRows or the row ResizeObserver.

Changelog

• Changed: Opening or closing a long prompt's Show more and a one-file edited card's diff in agent chat now animates and keeps the clicked line in place.

Frame strips

These are synthetic gallery fixtures, captured with headless Playwright on the fleet at 760x640 in the dark theme. Each fold's token is slowed tenfold to 1800 ms so one fold yields 8 to 12 frames. Each strip shows the open, then the close. Each frame's label gives ms since the click, the clicked header's y offset (dy), and the number of running animations (DUP would mark an element running two at once). Head is bf316ac; the commit after it changes only the motion-off hold.

Kind	Chromium head dy (open / close)	WebKit head dy	animations per element
Command group	0 / 0	0 / 0	1
Tool row	0 / 0	0 / 0	1
MCP call	0 / 0	0 / 0	1
Thought	0 / 0	0 / 0	1
One-file diff	0 / 0 (base: no animation)	0 / 0	1
Show more	bubble top held; button rides the edge (base: no animation)	same	1
Agent message card	0 / 0	0 / 0	1
Background task note	0 / 0	0 / 0	1

Show more's dy is the button's, which rides the bubble's growing edge by design. The gallery's prompt sits at the transcript's end, where closing lets the browser clamp the offset (the one documented exception, #23093). The browser test below checks Show less mid-transcript.

One-file diff, before (no animation; the diff appears and vanishes in one frame):

After, Chromium and WebKit:

Show more / Show less, before (no animation):

After, Chromium and WebKit:

The rest, after (Chromium; WebKit measured the same, table above):

Live side by side (tailnet): https://cmux-lawrences-mac-mini.tail137216.ts.net:18796/compare/tp-fold-animation/?entry=agent-pane.transcript

Observed / Not run

• transcript-fold-anchor-browser.test.ts (Chromium) adds the following:
  • One test per collapsible kind (a thought, a tool call, a command group, Show more). Each opens and closes mid-transcript, and checks on every painted frame that the line stays within 1 px and that no element runs more than one animation.
  • A 400-line prompt's Show less with Reduce Motion keeps its button on its pixel.
• PR CI's webviews job runs these. The fleet steps sat queued behind the suites label for more than 30 minutes, so I cancelled them.
• Reviewed by Codex (gpt xhigh, via acpmux) over several rounds. The receipt is in a comment at the head.
• CI reds that also fail on base:
  • cmux-next checks: "No fleet host names in gallery fixtures", from base files ShowcaseMachineNameTests.swift and ChatInspector.gallery.tsx.
  • cmux-next checks: "Checks job structure", from test_cmux_next_checks_workflow.py web-bundles. This PR touches no workflow.
  • Guard status and release-notary.
• Not run: an app build (this is a webview-only change).

Agent session: cosmic-gray

🤖 Generated with Claude Code

[github-actions]

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

Review 3a581d7: 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=pr24050 -f ref=tp/fold-animation -f platform=macos -f backend_mode=local

Passes: CI passes on 3a581d784c.

CI passes on 3a581d784c (run 39929998813 attempt 1).

Written by scripts/ci/classify_failures.py (ci-failure-attribution.yml); signatures are its SIGNATURES table. A machine verdict is the runner's fault, not this PR's; yours means the failing file is one this PR changes, also red on main that main's latest full suite fails the same way, seen on other PRs that it failed on another pull request's run lately.

red: cmux-next checks (god files, concurrency, crash safety, l10n); NEW: red check: Checks job structure (+2 more) is not red on the base 4654a98; do not merge until green; see lane-rules

Tour of 3a581d784c (run): no media. The tour did not run: only webview code changed (15 path(s)), which the gallery advisory run checks in Linux Chromium on the pull request (its artifact holds any captures); add the media-tour-mac label, then re-add media-tour, to tour it on a Mac.

Gallery before/after for 3a581d7: 16 states rendered, 8 images differ, 1660 more touched states not rendered. Images are in the run's artifact, not on this public PR.

gh run download 39929998345 -R manaflow-ai/cmux -n gallery-advisory-24050-3a581d784c02d7ac4e96c3707bd0ddea5764f21a (then read gallery-review.json)

States

State	Theme	Status	Diff	Picked by
agent-pane.background-task-note/on-call	Apple System Colors	changed	0.0124%	own BackgroundTaskNote.tsx
agent-pane.background-task-note/on-call	Apple System Colors Light	changed	0.0119%	own BackgroundTaskNote.tsx
agent-pane.background-task-note/lone	Apple System Colors Light	changed	0.0109%	own BackgroundTaskNote.tsx
agent-pane.background-task-note/lone	Apple System Colors	changed	0.0076%	own BackgroundTaskNote.tsx
agent-pane.background-task-note/ended	Apple System Colors Light	unchanged	0%	own BackgroundTaskNote.tsx
agent-pane.background-task-note/ended	Apple System Colors	unchanged	0%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/collapsed	Apple System Colors Light	unchanged	0%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/collapsed	Apple System Colors	unchanged	0%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/failed-open	Apple System Colors Light	changed	0.598%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/failed-open	Apple System Colors	changed	0.531%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/completed-open	Apple System Colors Light	changed	3.07%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/completed-open	Apple System Colors	changed	0.79%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/stopped-open	Apple System Colors Light	unchanged	0%	own BackgroundTaskNote.tsx
agent-pane.background-task-note-open/stopped-open	Apple System Colors	unchanged	0%	own BackgroundTaskNote.tsx
agent-pane.chat-width/standard	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.chat-width/standard	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.chat-width/wide	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.chat-width/wide	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.chat-width/wide-blocks	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.chat-width/wide-blocks	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.composer-floats/scrolled-to-end	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.composer-floats/scrolled-to-end	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.composer-floats/composer-grown	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.composer-floats/composer-grown	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.docked-chat/new-chat	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.docked-chat/new-chat	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.docked-chat/after-turn	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.docked-chat/after-turn	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.docked-chat/working	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.docked-chat/working	Apple System Colors	unchanged	0%	own App.tsx
agent-pane.docked-chat/typing-in-dock	Apple System Colors Light	unchanged	0%	own App.tsx
agent-pane.docked-chat/typing-in-dock	Apple System Colors	unchanged	0%	own App.tsx

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

[blacksmith-sh]

Found 8 test failures on Blacksmith runners:

Failures

Test	View Logs
/tmp/tmpqucwhh_y/summary-missing.json (live):/summary-missing.json (live):	View Logs
/tmp/tmpxo8gslws/summary-fail.json (live):/summary-fail.json (live):	View Logs
[tabs] debug.tab_switch timed 0 of 40 switches/ [tabs] debug.tab_switch timed 0 of 40 switches	View Logs
[tabs] debug.tab_switch timed 0 of 40 switches/ [tabs] debug.tab_switch timed 0 of 40 switches	View Logs
[tabs] the window draws no frames/[tabs] the window draws no frames	View Logs
[tabs] the window draws no frames/[tabs] the window draws no frames	View Logs
test_base_passes (main.NoticesPairTests.test_base_passes)/ test_base_passes (main.NoticesPairTests.test_base_passes)	View Logs
test_targets_outside_the_app_go_to_the_mac_canary (main.SwiftCanaryFleetCoverage.te st_targets_outside_the_app_go_to_the_mac_canary)/ test_targets_outside_the_app_go_to_the_mac_canary (main.SwiftCanaryFleetCoverage.te st_targets_outside_the_app_go_to_the_mac_canary)	View Logs

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

[teamleaderleo]

Bead cx-r65q2 created for this PR by the cmux coordinator.

[teamleaderleo]

Review (codex xhigh): SHIP at a6a13b7

No blocker or major findings. The round-two fix addresses the motion-off Show less virtualization failure by retaining the anchor row and synchronizing scroll state before paint. The full diff and the surrounding code were reviewed; no tests or builds were run.

The earlier rounds of this review (Codex xhigh via acpmux, on a different model from the author) found these issues, all fixed at this head or moved out of this PR with the Worked for row motion:

• headers drifting when a second fold started mid-slide;
• rows sliding against a reader's scroll;
• ghosts outliving a session switch;
• Show more animations that were never cancelled;
• virtualized rows snapping;
• O(rows) work per render.

[teamleaderleo]

Orphaned: no activity for 13 h, and no lane holds a lease on it. The orphan sweep could not merge feat-cmux-next in: conflicts in webviews/src/agent-session/acpmux/conversation/CommandRow.tsx, webviews/src/agent-session/acpmux/conversation/ToolGroupRow.tsx. Listed for a lane to resolve.

[cursor]

Bugbot needs on-demand usage enabled

Bugbot uses usage-based billing for this team and requires on-demand usage to be enabled.

A team admin can enable on-demand usage in the Cursor dashboard.

[teamleaderleo]

cmux-next / cmux-next checks (god files, concurrency, crash safety, l10n) › Checks job structure is red on 9 of 14 fresh feat-cmux-next PR heads in the last 2:00:00, so it is most likely a base red, not this PR's. The base-reds lane has it; a base merge picks up the fix once it lands.

[teamleaderleo]

cmux-next / cmux-next checks (god files, concurrency, crash safety, l10n) › No fleet host names in gallery fixtures is red on 9 of 14 fresh feat-cmux-next PR heads in the last 2:00:00, so it is most likely a base red, not this PR's. The base-reds lane has it; a base merge picks up the fix once it lands.

[teamleaderleo]

cmux-next guards / nightly-guards / workflow-guard-tests / release-notary is red on 19 of 20 fresh feat-cmux-next PR heads in the last 2:00:00, so it is most likely a base red, not this PR's. The base-reds lane has it; a base merge picks up the fix once it lands.

[teamleaderleo]

cmux-next / cmux-next checks (god files, concurrency, crash safety, l10n) › No new SwiftUI (macOS UI is AppKit) is red on 12 of 13 fresh feat-cmux-next PR heads in the last 2:00:00, so it is most likely a base red, not this PR's. The base-reds lane has it; a base merge picks up the fix once it lands.

[teamleaderleo]

rust workspaces / plan (changed Cargo workspaces) is red on 12 of 15 fresh feat-cmux-next PR heads in the last 2:00:00, so it is most likely a base red, not this PR's. The base-reds lane has it; a base merge picks up the fix once it lands.

[teamleaderleo]

rust workspaces / rust workspaces is red on 16 of 17 fresh feat-cmux-next PR heads in the last 2:00:00, so it is most likely a base red, not this PR's. The base-reds lane has it; a base merge picks up the fix once it lands.

[teamleaderleo]

Review (codex xhigh, correctness first): SHIP at 3a581d7

• Codex xhigh (codex exec --sandbox read-only on cmux16, diff-only, no checkout) reviewed the full gh pr diff at ad0ab9e and said SHIP. An earlier round at 430b2cd said HOLD over two points. First, rapid toggles could stack the fallback scroll corrections; that's fixed: a toggle or an unmount now cancels the running correction. Second, Find reveals don't hold their header; that's kept on purpose and documented, because a hold would undo Find's scroll to its match. A delta review of the final commit, which is test-only (the long-prompt test now waits for Show less's fold before measuring), said SHIP at 3a581d7.
• The merges of base resolved conflicts in imports only (CommandRow, McpCallRows, ToolRow, ToolRun), and the reviewer checked them.
• Ran on huge-blue at 3a581d7:
  • transcript-fold-anchor-browser: 8/8
  • fold-motion: 15/15
  • fold-no-blank-space-browser: 1/1
  • worked-fold: 9/9
  • ThoughtRow: 30/30
  • transcript-working-line: 12/12
  • agent-pane-long-prompt: 1/1
  • Lint and format are clean on the changed files. tsc reports errors only in QuestionCard.gallery.tsx and home-cloud-rows.test.ts, which this PR doesn't touch.
• Other reds at the previous head are not this PR's:
  • composer-type-scale (nt-style-input)
  • agent-pane-peer-chat
  • agent-pane-no-capsules
  • popup-one-hairline (.acpmux-omni)
  • rust workspaces, guard status, release-notary, and the cmux-next checks host-name and job-structure checks, which are also red on base.

[github-actions]

Shipped in:

• cmux NEXT nightly 1.0.0-nightly.3993394954401 (6ef5b51126)
• cmux NEXT nightly 1.0.0-nightly.3993797697001 (7e7162114e)