[teamleaderleo]

Focused new-tab motion fix on current feat-cmux-next base.

Validation: focused new-tab tests 10 passed, 31 assertions; diff-check clean.

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Summary by cubic

Stabilizes the new-tab list motion so it no longer remeasures row geometry or replays animations when the owner rerenders with the same row keys. The motion hook now keys off a string signature, a stable list root, and the row selector, and it re-observes a replaced root.

• Removed the explicit data-agent-motion="off" check; reduced-motion behavior now depends only on CSS variable overrides.
• Rerenders with identical row sets now skip all geometry reads for both normal and reduced-motion paths.

^{Written for commit 5a07094. Summary will update on new commits.}

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[teamleaderleo]

Bead cx-4790z created for this PR by the cmux coordinator.

[github-actions]

Unclear: web / react-apps-check (2) failed: no known signature; failed step: Test webviews.

CI failed on 5a07094f21 (run 39128708076 attempt 1): 2 unknown.

Job	Verdict	Why
web / react-apps-check (2)	unknown	no known signature; failed step: Test webviews
web / react-apps-check	unknown	no known signature; failed step: Check webview test shards

Not re-run automatically: web / react-apps-check (2), web / react-apps-check are not machine failures.

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.

[github-actions]

CI fast guards failed on 5a07094f21 (https://github.com/manaflow-ai/cmux/actions/runs/39128708076). It does not block the merge; a red guard merged into main breaks it for every open PR.
The log named no failed step; see the run.

Agents: python3 scripts/ci/guard_attribution.py fix applies the mechanical fixes locally. This comment is updated in place on each push.

[teamleaderleo]

Holding this one: it doesn't fix the workspace-switch replay Leo sees in the nightly, and it reverts hardening from merged #21476 and #21501:

• cancelRunning / trackAnimation are gone, so finished animations are never pruned from running and nothing cancels them on unmount.
• Both motion-off checks (isAgentMotionOff() and dataset.agentMotion === "off") are removed, so New Tab list motion ignores the host's motion-off setting.
• The layout effect now runs on every render, with no dependency list.

The one new idea, re-checking list.current when React swaps the root node under the same keys, could land on its own on top of the current listMotion.ts. Please don't open a v6 that rebases this diff over base again.

[teamleaderleo]

Closing: base already skips remeasuring when the row keys are unchanged and survives the Strict Mode effect replay. This diff would also undo two base fixes: #21476 (cancelling running animations) and #21501 (releasing finished ones), plus the motion-off check. #21781 is the current listMotion change.