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Size Change: -15 B (-0.02%) Total Size: 78.7 kB
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developit
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Mar 17, 2026
| hooks._pendingEffects.some(invokeCleanup); | ||
| hooks._pendingEffects.some(invokeEffect); | ||
| hooks._pendingEffects = []; | ||
| if (hooks._pendingEffects.length) { |
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Isn't this the wrong branch of this condition? The fix should be in the branch where currentComponent === previousComponent since it's for the case where the component is dirtied mid-render?
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This is in the right branch, when we immediately re-render due to an in-render setState we will reset the components hooks state. When we however have overlapping renders where we see that we have rendered this component before and it still has pending-effects to flush we will flush it. Now instead of flushing the component only we flush the entire subtree.
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The main reason for this is that nothing currently stops the following from happening
- Queue update
- Update runs, our root and an intermediary component enqueues an effect run
- Child queues an update for parent
- BEFORE: we flush the intermediary effect
- AFTER: we flush both intermediary as well as root effect
- intermediary renders again
Between 3 and 4 we would flush the parent updates
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Reproduction at https://stackblitz.com/edit/vitejs-vite-yxpjutvm?file=package.json - notice how the fade-out animation never triggers and instead it just dissapears
Problem
When libraries like
motion/react(framer-motion) and@base-ui/reactare used together with Preact/compat, exit animations break. The popup disappears instantly instead of animating out.The root cause is a timing issue: when a component re-renders before its
useEffectcallbacks have fired (viaafterPaint), Preact flushes only that component's stale effects duringoptions._render. This causes effects from different components to run in separate render cycles instead of as a single batch.In the concrete case:
useEffectflushes during one render cycle → callsgetAnimations()→ finds 0 animationsuseEffectflushes during a later render cycle → starts WAAPI animation → too latehidden→ popup disappearsHow React handles this
React calls
flushPendingEffects()at the start ofperformSyncWorkOnRootbefore beginning any new render work. This flushes all pending passive effects (from all components) as a batch, not just the re-rendering component's effects.From React's
ReactFiberRootScheduler.js:This guarantees that all
useEffectcallbacks from render N complete before render N+1 begins, regardless of which component is re-rendering.What Preact did before
In
options._render, when a component re-renders with unflushed effects (previousComponent !== currentComponent):This meant effects from different components could be spread across multiple render cycles when intermediate
setStatecalls triggered cascading re-renders.The fix
Replace the per-component flush with a call to
flushAfterPaintEffects(), which drains the entire pending effects queue:This matches React's approach: before starting new render work, ensure all effects from previous renders have completed.
Behavioral differences from the old code
diffed)afterPaint