# Reaching /mix from a project

Six of seven projects used to resolve to `null` in the mix bench: `MEDIA_ROOTS`
was the um-song subtree, so no report video's media was openable at all.

## The link

```
/mix?body=<absolute>&start=<s>&end=<s>&from=<projectId>&clip=<clipId>
```

- **`body` is absolute**, like the picker's own `<option value>`. That is what
  dodges the relative-binding hazard: a relative path is tried against each root
  in order and never stats, so it binds to the first root it *could* live under.
- **`from` and `clip` are provenance** — the back-crumb and the header line. They
  are deliberately not used to resolve media; that would be a second, divergent
  resolver for what the first one already did.
- A deliverable link carries no window: the whole thing is the point.

Resolved **server-side** in `app/mix/page.tsx`, which is why this needs no
`useSearchParams` and no Suspense boundary — that page already ran on the server,
it simply never read its own `searchParams`.

## What a clip opens, and why

The **widest cached raw window**, not the built segment.

1. It exists as soon as the clip has been fetched once; segments only exist after
   a build.
2. It has **no chrome burned in** — which is what a mix is looking at.
3. It is the file the bench already has peaks for.

The built segment is the fallback, and the link says which it is. A clip with
neither renders **no link at all** rather than a dead one that 400s.

## The prefill

`start = clip.start - fetchStart`, `end = clip.end - fetchStart`, computed
server-side because the arithmetic is exact and known there; making the client do
it would be a second place to get it wrong.

The bench then says *"this window came from c01 — the file itself runs
0.00–9.00s"*, so the material outside the window does not look unreachable.
Dragging past the end is harmless: `resolveMix` clamps `end` to the body's
duration, and `normaliseSpec` clamps `start` to ≥ 0.

## Precedence

**preset > the saved pair > blank.** A link that named a file and a window must
not lose to whatever the bench was last pointed at. The per-pair knobs — handover,
fade, gain, duck — *are* adopted, because those are things learned about that
pair.

Arriving via a link does **not** write a session. A session is saved only on a
real render, so drive-by navigation cannot change what the next bare `/mix` visit
opens.

## Refuse, never clamp

A `body` outside the roots, or one with no audio track, produces a bench with no
preset and a **visible reason**. Silently opening a different file than the link
named is the one outcome worse than an error, and `lib/mix.ts` already takes this
line for the same reason.

## The picker

Grouped by project via `<optgroup>` — zero client JS, still a native select — with
a one-line text filter above it.

Coverage is a property of the **enumeration**, not of the cap: each project is
asked for its own files with its own small cap, so one busy project cannot push
every other off the end. Measured before this: four of six report deliverables
fell off a 600-entry newest-first list, and per-song cuts never appeared at all.

A report project whose only media is `out/clips-raw` correctly offers **nothing** —
those are intermediates, excluded by name.

## Where a mix lands

`SONG_REPORTS`, still. Reports became **readable**, not writable — see
[folders.md](folders.md). A mix of a report clip therefore lands in the um-song
deliverables directory, which is not ideal, and the trade was deliberate: the
alternative made every report's `out/` a legal render target, one typo away from
overwriting a 46 MB deliverable that cost an hour of fetches.