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# Renderer performance probes

Measuring what a session switch costs in the running Desktop app, over CDP. The
findings in #4109 were produced with these; they live here so the next
measurement is a command rather than a rebuild.

They are ad-hoc tools, not a benchmark suite: they attach to a dev app you
started, and they answer "how much" only for the interaction they drive.

## Running

Start the dev app with the debugger open:

```bash
npm run dev -w @maka/desktop -- --remote-debugging-port=9334
```

Then, with a populated sidebar (about 30 rows is what the #4109 numbers used):

```bash
node scripts/perf/session-switch-commits.mjs before
node scripts/perf/session-switch-busy-js.mjs
```

- `session-switch-commits.mjs` — React commits per switch, how many are
  full-tree renders, and how many fibers actually re-rendered. Reloads the page
  first, so `react-commit-probe.js` is in place before React boots.
- `session-switch-busy-js.mjs` — renderer busy JS per switch, with the top
  self-time entries. It reports the running build, one configuration. A
  before/after needs both configurations in one instance; see below.
- `react-commit-probe.js` — the in-page half: a minimal
  `__REACT_DEVTOOLS_GLOBAL_HOOK__` plus a `Function.prototype.bind` wrapper that
  catches React creating a `dispatchSetState`, so a commit can be attributed to
  the `setState` that caused it.
- `cdp-client.mjs` — the protocol client and the row-clicking helper.

## The one rule

**Never compare numbers from two app launches.** Restarting the app shifts these
metrics by orders of magnitude, while the spread inside a single running
instance is small. An A/B means: one instance, a `globalThis` switch to select
the configuration, at least three repetitions per configuration, alternating,
compared pairwise. Two runs of "before" and "after" against two launches will
produce a confident number that means nothing.

The switch is a throwaway, not a shipped affordance. Branch a worktree, put a
`globalThis` flag in the one place that reads it — one that restores the old
behaviour when set, so the regressed configuration is reachable without
rebuilding the defect by hand — alternate the flag inside the running instance,
then delete the worktree. Measurement discipline is ours; product code carries
none of it. A flag that reaches `main` is a branch nothing executes and a
defect replica that has to be maintained against the code it replicates.

Two smaller ones that cost time to rediscover:

- Clicks must be `Input.dispatchMouseEvent`. A synthesised `element.click()`
  does not activate an Astryx `SideNavItem`, so the app does nothing and the
  measurement describes it.
- Disarm the commit probe (`__MAKA_PROBE__.arm = false`) before sampling. Its
  fiber walk otherwise shows up in the profile as the app's own work;
  `session-switch-busy-js.mjs` does this for you.

## What these do not tell you

The commit count is unweighted, and busy JS on this workload is spread across
about a thousand cheap fibers rather than concentrated in one component. A
change that lowers the commit count has not necessarily lowered the time, and a
change that moves the top self-time entry has not necessarily lowered anything.
Read both, and read the medians.