fix: events not firing when SpringRef attached manually under StrictMode#2430
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Closes #1991 flushUpdate mutates the input props in place by wrapping event handlers (onStart/onChange/onRest) with a batching closure. useSprings' commit-phase layout effect pushes updates.current[i] — a stable reference reused across renders — onto ctrl.queue. Under StrictMode the layout effect runs twice, so the already-wrapped handler from the first flush gets wrapped again. The outer wrapper now closes over wrapper1 instead of the user's callback, so when _onFrame flushes the event Map it only ever invokes wrapper1 — which just adds itself to the queue. The user's handler is never reached. Push a shallow copy of the update (including its default sub-object) onto ctrl.queue so flushUpdate's mutations land on the copy and never leak back into the canonical updates.current[i] reference.
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This was referenced May 21, 2026
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Closes #1991.
Summary
When a user attaches a
SpringRefmanually via therefprop onuseSpring/useSprings, events (onStart,onChange,onRest) silently stop firing under React StrictMode, even though the animation itself still plays.Root cause
flushUpdateinController.tsmutates the input props in place — it wraps each event handler (onStart,onChange,onRest) with a batching closure so multiple per-frame invocations collapse into one. The wrapper captures the user's original handler in its closure.useSprings' commit-phase layout effect pushesupdates.current[i](a stable reference reused across renders) ontoctrl.queuewhenever a ref is attached. Under StrictMode the layout effect runs twice (mount → cleanup → re-mount), so the already-wrapped handler from the first flush gets wrapped a second time. The outer wrapper now closes overwrapper1instead of the user's callback, so when_onFrameflushes the event Map it only ever invokeswrapper1— which just adds itself to the queue. The user's handler is never reached.Fix
Push a shallow copy of the update (including its
defaultsub-object) ontoctrl.queue, soflushUpdate's mutations land on the copy and never leak back into the canonicalupdates.current[i]reference.One-line change in
packages/core/src/hooks/useSprings.ts.