fix(core): preserve velocity across retargets for decay animations#2516
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Decay animations integrate an initial velocity; the previous `_update` logic unconditionally reset `config.velocity` to `0` whenever `to` or `from` was provided, silently breaking gesture-driven decay flows that retarget mid-throw. Skip the reset when `config.decay` is set. Also clarifies the JSDoc and docs-site reference for `decay` to explain that it decelerates from an initial velocity and does not ease toward `to` — the root cause of the user-facing confusion in #1843. Closes #1843.
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Summary
Decay animations integrate an initial velocity —
position = from + (v0 / (1 - decay)) * (1 - e^(-(1-decay)t)). The previous_updatelogic unconditionally resetconfig.velocity = 0whenevertoorfromwas provided, which silently breaks gesture-driven decay flows that retarget mid-throw (mouse-flick decay, drag-throw, etc.). The fix is a one-condition tweak: skip the velocity reset whenconfig.decayis set.The bug report (#1843) was filed because
useSpring({ xys, config: { decay: 0.48 } })"doesn't work". That repro never sets a velocity at all, so even after this fix it won't animate — decay needs a velocity to decelerate. The user-facing confusion is that the existing docs don't say so. This PR also updates thedecayJSDoc and docs-site reference to make that explicit, including a@use-gesture/reactsnippet that mirrors therocket-decaydemo.Closes #1843.