[Transition] Mount child in same commit when opening from unmounted#48649
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siriwatknp merged 3 commits intoJun 11, 2026
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When opening from unmounted, the child was mounted in a layout effect, adding a commit where it was still null. Consumers reading a ref right after `in` flips (e.g. focus management in the vertical stepper demo) crashed. Restore react-transition-group's behavior of deriving the status during render so the child and its ref are present in the same commit.
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Tighten regression test: compare ref to mounted node instead of !== null, and assert onExited stays silent during the intermediate exited status.
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This seems a better way than mine #48646 |
mj12albert
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Jun 10, 2026
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Looks good ~ it fixes https://deploy-preview-48649--material-ui.netlify.app/material-ui/react-stepper/
siriwatknp
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closes #48637
Summary
Fixes the
VerticalLinearStepperdemo crash introduced by the customTransitioncomponent (#48325).Root cause
When opening from
unmounted, the newinternal/Transitionmoved the child fromunmounted → exitedinside a layout effect, adding a commit where the child is stillnull.react-transition-groupderived that status from props during render, so the child existed in the same commitinflipped true.The stepper demo focuses a button synchronously in a passive effect right after
activeStepchanges. With the extra commit, the previous step's button is already unmounted and the new one isn't mounted yet, socontinueButtonRef.currentisnulland.focus()throws.This is a silent behavior regression for any consumer that reads a ref (focus management, measurement, scroll-into-view) right after
intoggles — not just this demo.Fix
Move the
unmounted → exitedtransition to a guarded render-phase update so the child is committed (and its ref attached) in the same commitinturns true, matchingreact-transition-group. The redundantunmountedbranch is removed from the reconcile effect. No demo change needed.Testing
Added a regression test mirroring the demo's exact pattern (a parent passive effect reads the child ref right after
inflips). It fails (null) without the fix and passes with it.Verified green:
Transition,Collapse,StepContent,Fade,Grow,Zoom,Slide,Accordion,Snackbar,Tooltip,Dialog,Drawer,Menu,Popover,SpeedDial,TouchRipple,Ripple.