fix(core): expose new phase to useTransition render fn#2507
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Previously `t.phase` was only mutated to the new phase inside a useIsomorphicLayoutEffect, so the render fn always saw the previous phase during the render that triggered the leave animation. The next render that would have surfaced `'leave'` was suppressed because the transition had already expired and been pruned, leaving consumers unable to branch on `state.phase === 'leave'`. Read the upcoming phase from the changes map (or exitingTransitions when exitBeforeEnter is set) and pass a shallow copy of the transition state with that phase to the render fn. No state mutation during render; no API change. Closes #1654.
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This was referenced May 22, 2026
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Summary
In
useTransition, the render fn's third argument exposesstate.phaseso callers can branch on lifecycle (e.g.state.phase === 'leave'). Previously this phase was only mutated inside a layout effect that runs after render, so during the render that triggered the leave animation the consumer always saw the stale'enter'. By the time a follow-up render could have surfaced'leave', the transition had been marked expired and pruned — so'leave'was effectively unreachable for normal removals, only briefly observable when an item was re-added mid-leave.Fix
In the render loop, read the upcoming phase from the
changesmap (orexitingTransitionswhenexitBeforeEnteris set) and pass a shallow copy of the transition state with that phase to the render fn. No state mutation during render; no API change; the existing layout effect still owns the canonical mutation.Regression test in
useTransition.test.tsxremoves an item and asserts the render fn observedphase === 'leave'at some point during a normal removal — fails onnext, passes with this change.Closes #1654.