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perf(tui): instrument scroll fast-path decline reasons

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Adds scrollFastPathStats counters to render-node-to-output.ts: captures
every time a ScrollBox's DECSTBM scroll hint is generated, records
whether the fast path took it (blit+shift from prevScreen) or declined,
and why. Exposed through hermes-ink's public exports and snapshotted on
every FrameEvent so the profiler harness can correlate decline reasons
with the actual patch/renderer cost per frame.

This is pure observation — no behaviour change. Preparing for the
virtual-history rewrite: the hypothesis was that our topSpacer/
bottomSpacer scheme disqualifies every scroll via heightDelta
mismatch, but the data shows the fast path is actually taken on most
scrolls (19/23 over a 6s PageUp hold through 1100 messages) — the
remaining steady-state renderer cost is Yoga tree traversal, not
the per-frame full redraw I initially suspected.

Declines that do happen correlate with React commits that changed the
mounted range mid-scroll (heightDelta=±3 to ±35). Those are the rarer
cases the virtualization rewrite still needs to address.

No test diffs — instrumentation-only.  Build verified: `tsc --noEmit`
plus the full `npm run build` compiler post-pass pass cleanly.
This commit is contained in:
Brooklyn Nicholson 2026-04-26 16:45:53 -05:00
parent 71eee26640
commit cd7a200e6c
4 changed files with 114 additions and 0 deletions
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22
ui-tui/src/lib/perfPane.tsx
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@ -44,6 +44,7 @@ import { homedir } from 'node:os'
import { dirname, join } from 'node:path'
import type { FrameEvent } from '@hermes/ink'
import { scrollFastPathStats } from '@hermes/ink'
import { Profiler, type ProfilerOnRenderCallback, type ReactNode } from 'react'
const ENABLED = /^(?:1|true|yes|on)$/i.test((process.env.HERMES_DEV_PERF ?? '').trim())
@ -122,8 +123,29 @@ export const logFrameEvent = ENABLED
return
}
// Snapshot the fast-path counters each frame. Cumulative values —
// consumers diff pairs to get per-frame deltas. Written verbatim
// so we can also see "last*" fields (which decline reason fired,
// and what the height math looked like).
const fastPath = {
captured: scrollFastPathStats.captured,
taken: scrollFastPathStats.taken,
declined: {
heightDeltaMismatch: scrollFastPathStats.declined.heightDeltaMismatch,
noHint: scrollFastPathStats.declined.noHint,
noPrevScreen: scrollFastPathStats.declined.noPrevScreen,
other: scrollFastPathStats.declined.other
},
lastDeclineReason: scrollFastPathStats.lastDeclineReason,
lastHeightDelta: scrollFastPathStats.lastHeightDelta,
lastHintDelta: scrollFastPathStats.lastHintDelta,
lastPrevHeight: scrollFastPathStats.lastPrevHeight,
lastScrollHeight: scrollFastPathStats.lastScrollHeight
}
writeRow({
durationMs: round2(event.durationMs),
fastPath,
flickers: event.flickers.length ? event.flickers : undefined,
phases: event.phases
? {