NG0951 thrown when deleting a custom interactor resource

[adamjohnwright]

Deleting a resource added through the custom interactor dialog throws an Angular
runtime error into the console. The list and the diagram end up correct — the
assertion that fails is the one checking nothing threw on the way.

Caught by e2e/custom-interactor-dialog.spec.ts:269, "Deleting a resource you
added › removes it from the diagram, not just from the list":

Error: without throwing on the way out

expect(received).toEqual(expected)
- Array []
+ Array [
+   "RuntimeError: NG0951: Child query result is required but no value is available."
+ ]

What it is

NG0951 means a viewChild.required() / contentChild.required() was read while
its target was not in the view. Something is reading a required query during the
teardown that deleting a resource causes.

What it is not

• Not viewport.component.ts:212 (interactors = viewChild.required(InteractorsComponent))
  as far as I can tell — cr-interactors is not inside a conditional block. It is
  always rendered and merely hidden with [style.display], so that query has a
  value throughout.
• Not caused by the e2e fixture work; it reproduces during recording against a
  live backend.

The remaining required queries are listed by
grep -rn "viewChild.required" projects/pathway-browser/src/app/ — the ones worth
looking at first are those in components that appear and disappear with the
interactors panel, since deletion is what triggers it.

Why it matters

It is a thrown error on an action a curator will take. The visible behaviour is
currently correct, which makes it the kind of thing that gets dismissed as noise
until it becomes a real failure after an unrelated change — a required query with
no value is one refactor away from breaking the panel outright.

The test asserting "nothing threw" is doing exactly its job here; the value of
that assertion is that it fails while the symptom is still invisible.

[adamjohnwright]

Cause still unknown. Recording what has been ruled out so the next person does
not repeat it.

Not reproducible on demand

The test that caught it — custom-interactor-dialog.spec.ts:269, which asserts
nothing threw — is clean under every condition I could construct:

• five consecutive runs of that test alone
• the whole spec (9 tests)
• 37 tests across the interactor-heavy specs on four workers

It has surfaced twice, both times during a full 154-test run: once while
recording against a live backend, once on a fixtures verification. So it needs
whole-suite load, not this spec.

The candidate I found, and why it is only a candidate

NG0951 means a required query was read while its target was not in the view.
Rather than keep chasing an intermittent, I asked which required queries point
at something that can be absent when read. Two in the whole pathway browser, and
one was read from a callback detached from the lifecycle:

ngAfterViewInit(): void {
  setTimeout(() => { this.darkToggle()._switchElement... }, 200);
}

A timer firing after teardown reads a required query with no value, which is
exactly the error. But I could not make it throw. A probe that mounts the
viewport and tears it down six times inside the 200ms window produced zero errors
with the old code in place.

That probe was also flawed, which is worth knowing before anyone rebuilds it: it
navigated to /about, which lives in the website-angular app, while
ViewportComponent is a route in the pathway-browser's own router. That is a
full page load, so the timer died with the JS context instead of outliving a
destroyed component. Destroying it properly means navigating to another route
inside the pathway-browser app.

Changed anyway in #239, as hardening rather than a fix: afterNextRender cannot
run after destruction and waits for a real render instead of guessing 200ms.

What would actually catch it

The assertion that found it is the right detector — it is a test asserting the
console stayed clean, which is why this was visible at all. What is missing is
where: NG0951 names no query, no component and no file.

Two things that would make the next occurrence diagnosable:

1. A trace on first failure. playwright.config.ts sets
   trace: 'on-first-retry', so the run that fails first has no trace. For an
   error this rare, the first occurrence is the only one you get.
2. The other required query. viewport.component.ts:212,
   interactors = viewChild.required(InteractorsComponent). cr-interactors is
   not inside an @if — it is always rendered and merely hidden with
   [style.display] — so it looked safe and I did not pursue it. Worth a second
   look by someone who has not already convinced themselves.

[adamjohnwright]

Reopened. This was closed automatically by the commit message on #239, which still
said "Closes #231" after I had corrected the pull request body to "Refs" — GitHub
honours the commit, not the description.

The cause is still unknown. #239 replaced a detached setTimeout with
afterNextRender, which removes one plausible way to produce NG0951 and is
better practice regardless, but I could not make the old code throw and said
so on the PR. Closing it as completed would leave the next person believing this
was diagnosed.

What is ruled out, and the two things that would make the next occurrence
diagnosable, are in the comment above.