Refactor ReactProfilerTimer to Track Transition Timers Per Lane

[subhamkumarr]

Summary

This PR refactors ReactProfilerTimer.js to correctly track transition timers on a per-lane basis, addressing a long-standing TODO (// TODO: This should really be one per Transition lane.).

Previously, transition timings (start time, update time, event time) were stored in global variables (transitionStartTime, transitionUpdateTime, etc.). This caused issues when multiple transitions were being processed concurrently or interleaved, as the global state could be overwritten, leading to inaccurate profiling data.

Key Changes

packages/react-reconciler/src/ReactProfilerTimer.js

• Replaced global transition timer variables with a transitionTimers Map, keyed by Lane.
• Introduced WeakMaps (pendingTransitionStartTimes, pendingTransitionEventInfo) to buffer timing data associated with a Transition object before a specific Lane is assigned.
• Updated startUpdateTimerByLane, startHostActionTimer, startAsyncTransitionTimer, and other helper functions to read/write from the transitionTimers Map instead of global variables.
• Added getTransitionTimers(lane) to safely retrieve timer data for a specific lane.

packages/react-reconciler/src/ReactFiberWorkLoop.js

• Updated the includesTransitionLane block in the work loop logic.
• It now iterates through all active transition lanes using getHighestPriorityLane.
• For each lane, it retrieves the corresponding timer data and logs the transition start/update.
• Ensures clearTransitionTimer(lane) is called for the specific lane being processed, rather than a global clear.

Verification

Automated Tests

• Passed yarn test packages/react-reconciler/src/__tests__/ReactTransitionTracing-test.js
• Passed yarn test packages/react-reconciler/src/__tests__/ReactPerformanceTrack-test.js

Manual Verification

• Verified that no regressions were introduced in basic profiling behavior.

Related Issues

Fixes TODO in ReactProfilerTimer.js.

[eps1lon]

Can you think of some tests to add (e.g. in ReactTransitionTracing-test.js) that show the difference with the new implementation?

[subhamkumarr]

Can you think of some tests to add (e.g. in ReactTransitionTracing-test.js) that show the difference with the new implementation?

Done! I added a regression test in ReactTransitionTracing-test.js called should correctly track start times for interleaved transitions.

It starts two transitions (at 1000ms and 2000ms) on separate roots and verifies they complete with their distinct start times, confirming they don't overwrite each other's global state.

[aydenhailey78-hub]

Okay that works

…

On Tue, Feb 10, 2026 at 4:51 PM subham kumar ***@***.***> wrote: Summary This PR refactors ReactProfilerTimer.js to correctly track transition timers on a per-lane basis, addressing a long-standing TODO (// TODO: This should really be one per Transition lane.). Previously, transition timings (start time, update time, event time) were stored in global variables (transitionStartTime, transitionUpdateTime, etc.). This caused issues when multiple transitions were being processed concurrently or interleaved, as the global state could be overwritten, leading to inaccurate profiling data. Key Changes packages/react-reconciler/src/ReactProfilerTimer.js - Replaced global transition timer variables with a transitionTimers Map, keyed by Lane. - Introduced WeakMaps (pendingTransitionStartTimes, pendingTransitionEventInfo) to buffer timing data associated with a Transition object before a specific Lane is assigned. - Updated startUpdateTimerByLane, startHostActionTimer, startAsyncTransitionTimer, and other helper functions to read/write from the transitionTimers Map instead of global variables. - Added getTransitionTimers(lane) to safely retrieve timer data for a specific lane. packages/react-reconciler/src/ReactFiberWorkLoop.js - Updated the includesTransitionLane block in the work loop logic. - It now iterates through all active transition lanes using getHighestPriorityLane. - For each lane, it retrieves the corresponding timer data and logs the transition start/update. - Ensures clearTransitionTimer(lane) is called for the specific lane being processed, rather than a global clear. Verification Automated Tests - Passed yarn test packages/react-reconciler/src/__tests__/ReactTransitionTracing-test.js - Passed yarn test packages/react-reconciler/src/__tests__/ReactPerformanceTrack-test.js Manual Verification - Verified that no regressions were introduced in basic profiling behavior. Related Issues Fixes TODO in ReactProfilerTimer.js. ------------------------------ You can view, comment on, or merge this pull request online at: #35759 Commit Summary - 06bebe9 <06bebe9> Refactor ReactProfilerTimer to track transition timers per lane File Changes (3 files <https://github.com/facebook/react/pull/35759/files>) - *M* packages/react-reconciler/src/ReactFiberWorkLoop.js <https://github.com/facebook/react/pull/35759/files#diff-72ff2175ae3569037f0b16802a41b0cda2b2d66bb97f2bda78ed8445ed487b58> (307) - *M* packages/react-reconciler/src/ReactProfilerTimer.js <https://github.com/facebook/react/pull/35759/files#diff-c69c604b5395aa2edacdbb0665a529e3f483f02e5b2eb7a5e6ea063e5a60349e> (247) - *M* packages/react/src/ReactStartTransition.js <https://github.com/facebook/react/pull/35759/files#diff-adb3f77f9697515adf24f3f32e9c3429635ca41d280f577a753ec4dbd0fa2fe0> (10) Patch Links: - https://github.com/facebook/react/pull/35759.patch - https://github.com/facebook/react/pull/35759.diff — Reply to this email directly, view it on GitHub <#35759>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/B4CX3QANRCSDLJOPN5YDJ5L4LID6LAVCNFSM6AAAAACUULYF2GVHI2DSMVQWIX3LMV43ASLTON2WKOZTHEZDENBRG4ZTCNY> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[djhdusjbd-eng]

Thanks

…

On Tue, Feb 10, 2026 at 4:52 PM subham kumar ***@***.***> wrote: Summary This PR refactors ReactProfilerTimer.js to correctly track transition timers on a per-lane basis, addressing a long-standing TODO (// TODO: This should really be one per Transition lane.). Previously, transition timings (start time, update time, event time) were stored in global variables (transitionStartTime, transitionUpdateTime, etc.). This caused issues when multiple transitions were being processed concurrently or interleaved, as the global state could be overwritten, leading to inaccurate profiling data. Key Changes packages/react-reconciler/src/ReactProfilerTimer.js - Replaced global transition timer variables with a transitionTimers Map, keyed by Lane. - Introduced WeakMaps (pendingTransitionStartTimes, pendingTransitionEventInfo) to buffer timing data associated with a Transition object before a specific Lane is assigned. - Updated startUpdateTimerByLane, startHostActionTimer, startAsyncTransitionTimer, and other helper functions to read/write from the transitionTimers Map instead of global variables. - Added getTransitionTimers(lane) to safely retrieve timer data for a specific lane. packages/react-reconciler/src/ReactFiberWorkLoop.js - Updated the includesTransitionLane block in the work loop logic. - It now iterates through all active transition lanes using getHighestPriorityLane. - For each lane, it retrieves the corresponding timer data and logs the transition start/update. - Ensures clearTransitionTimer(lane) is called for the specific lane being processed, rather than a global clear. Verification Automated Tests - Passed yarn test packages/react-reconciler/src/__tests__/ReactTransitionTracing-test.js - Passed yarn test packages/react-reconciler/src/__tests__/ReactPerformanceTrack-test.js Manual Verification - Verified that no regressions were introduced in basic profiling behavior. Related Issues Fixes TODO in ReactProfilerTimer.js. ------------------------------ You can view, comment on, or merge this pull request online at: #35759 Commit Summary - 06bebe9 <06bebe9> Refactor ReactProfilerTimer to track transition timers per lane File Changes (3 files <https://github.com/facebook/react/pull/35759/files>) - *M* packages/react-reconciler/src/ReactFiberWorkLoop.js <https://github.com/facebook/react/pull/35759/files#diff-72ff2175ae3569037f0b16802a41b0cda2b2d66bb97f2bda78ed8445ed487b58> (307) - *M* packages/react-reconciler/src/ReactProfilerTimer.js <https://github.com/facebook/react/pull/35759/files#diff-c69c604b5395aa2edacdbb0665a529e3f483f02e5b2eb7a5e6ea063e5a60349e> (247) - *M* packages/react/src/ReactStartTransition.js <https://github.com/facebook/react/pull/35759/files#diff-adb3f77f9697515adf24f3f32e9c3429635ca41d280f577a753ec4dbd0fa2fe0> (10) Patch Links: - https://github.com/facebook/react/pull/35759.patch - https://github.com/facebook/react/pull/35759.diff — Reply to this email directly, view it on GitHub <#35759>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/B4CX4CFUP2GLZI6FUCD6SED4LID6PAVCNFSM6AAAAACUULYF2GVHI2DSMVQWIX3LMV43ASLTON2WKOZTHEZDENBRG4ZTCNY> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[subhamkumarr]

@eps1lon
Is this Ok?

[subhamkumarr]

@hoxyq @eps1lon @unstubbable @jackpope
could anyone please review this and merge it when you get a chance? Thanks!
Fixes #35757