cmux NEXT agent pane: opening another chat plays the stagger motion by default
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-inor 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 #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
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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).