[teamleaderleo]

Leo, 2026-11-03, sharing a game menu animation clip: "i thought was cute and cool and i wanted to have more... nice stuff? around the app..." (and 2026-11-01: "honestly i just want to be able to be able to eventually play full-on animations like this").

Leo-ask: ask-1ec99cec90

What

The chat-switch-motion experiment (cmux#30602) now defaults to its picked arm, stagger: opening another chat moves the title, then the visible rows bottom-up (6 px rise, 16 ms apart, one spring) with Session Info's body following from the right, all settled within 240 ms. Off, Fade and Depth stay one cmux.experiments override away, and the gallery's compare arms are unchanged.

Spec: leo-workspace design/motion-and-polish.md (Choreography) carried the open question of whether a switch performance on by default outranks rule 14 ("instant if cached"). Leo's ask answers it. The switch itself stays instant: the new chat is in the DOM on the frame it lands, and the motion plays over it.

Risk

Small: a one-line default change plus comments. Already covered by what #30602 shipped:

• Reduce Motion: fades only, no travel; data-cmux-motion=off, a zero --agent-motion-in or a hidden page plays nothing.
• No timer or loop: finite Web Animations with fill: backwards, cancelled when the transcript unmounts; the next switch starts from the drawn value.
• Frame cost (latency harness chat.switch.<arm>, Linux pool, 10 runs, from cmux NEXT agent pane: chat switch motion experiment (rows, title and Session Info as one movement) #30602):

metric	before (off)	after (stagger)	delta	harness
frame p50	16.9 ms	16.7 ms	-0.2 ms	chat.switch.off / chat.switch.stagger
frame p95	36.7 ms	30.1 ms	-6.6 ms	same
dropped frames	10	11	+1	same
long tasks	0	0	0	same

Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Makes the stagger motion the default when opening another chat in the NEXT agent pane. The switch stays instant — the new chat is in the DOM on the frame it lands — and the motion plays over it; off and the other arms remain available via a cmux.experiments override ({"chat-switch-motion":"off"}).

• Motion still respects Reduce Motion, data-cmux-motion=off, a zero --agent-motion-in, or a hidden page.
• Frame cost is flat (p95 improves 6.6 ms; no new long tasks).

^{Written for commit 8bdc61c. Summary will update on new commits.}

[teamleaderleo]

Closing without landing. The independent review at 8bdc61c found that turning the stagger arm on by default breaks webviews/test/chat-open-perf.test.ts. Every open there is a switch, so stagger's 6 px row rise counts as a layout shift (line 739), and the cold-open fade is replaced (lines 752-758). The other arms (fade, depth) animate cached opens instead, which the same test rules out (line 763, 'instant if cached'). Any on-by-default arm changes a stated no-layout-shift / instant-if-cached contract, which needs an explicit decision rather than a default flip here. The experiment stays available via cmux.experiments.

[github-actions]

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

Review 8bdc61c: 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=pr33594 -f ref=lane/chat-switch-stagger-default -f platform=macos -f backend_mode=local