measure-rerenders.mts counts React commits on an untouched board and attributes each
to the shallowest component that re-rendered, i.e. where the update entered the tree.
What it found: an idle board commits 73-96 times per minute, re-rendering
@mantine/core/Box over 30,000 times in 60s. That is the churn behind the 650 MB
browser peak — transient allocation rather than retention. About 80% of those commits
enter at the same anonymous depth-2 provider, props [value, children], value {appDir}:
Next.js's internal HeadManagerContext, whose value is a fresh object literal on every
App Router render, so each render re-renders the whole app beneath it. The rest enter
at __next_metadata_boundary__ / <title>, pointing the same way.
The trigger is inside Next's router/metadata handling — application code has no
polling router.refresh() — so nothing here fixes it. It is the next thing to chase and
it dominates everything downstream of it.
useTimeAgo called setState on a 1s interval even when the label was unchanged ("3
minutes ago" holds for a full minute), re-rendering its subtree ~60x/minute per
consumer for an identical string. It now compares before setting. Recorded as a
correctness fix and NOT a measured win: idle commits stayed inside their 73-96 range
afterwards, because the app-root churn dominates.
pnpm test 1794/1795 (pre-existing pi-hole Testcontainers flake); typecheck clean
across 37 packages; no new lint errors.
Thomas Camlong
63e65bbc6c