[cpsievert]

Fixes #3286.

anywidget evaluates a widget's _esm module once per widget model (each model gets a fresh blob-URL import), so module-scope side effects in perspective-anywidget.js run once per model while customElements and globalThis remain page-global. Two module-scope side effects threw on the second evaluation, so any page with two PerspectiveWidget models — or one re-created model, e.g. re-running a notebook cell — rendered nothing:

• import "regular-layout" self-registers regular-layout, regular-layout-frame and regular-layout-tab unguarded → second evaluation throws NotSupportedError on customElements.define.
• Since Better WASM init #3269, module-scope perspective.init({ viewer, server }) re-registers against the page-wide wasm registry → second evaluation throws "viewer wasm is already initialized" (as an unhandled rejection, since index.js doesn't await it, leaving ready permanently rejected).

This PR completes the coexistence work started in #3192 (which guarded Perspective's own elements):

• perspective-viewer.ts — replace the static import "regular-layout" with a customElements.get() guard: await import("regular-layout") when unregistered (top-level await keeps registration synchronous with module evaluation, as the static import was), skip-and-warn otherwise, matching Add @perspective-dev/anywidget for vscode/jupyter #3192's bootstrap pattern. Done at the source rather than as an anywidget-build alias because the viewer build inlines regular-layout — this covers every consumer of the viewer bundle.
• registry.ts — in user mode, re-registering a consumed key that already has a source is now idempotent: keep the existing runtime, warn once per key per page, resolve. replace_server (init_server()) still throws for consumed keys, and keys consumed without ever being registered (e.g. server32 after server was selected) still throw as before.

Notes for reviewers

• The await import makes the viewer ESM dist an async module (top-level await). All in-repo consumers are ESM/es2022 or webpack 5, so this is legal; a hypothetical classic-script consumer of the ESM dist would break (none exists in-repo).
• Semantics worth a look: re-registering an unconsumed key still overwrites its source (pre-existing reconfiguration behavior), so a second anywidget evaluation silently refreshes the server source while the consumed viewer key skips+warns.

Validation

• registry.spec.js: 3 new tests (second init() resolves with an object-identical runtime; warn-once-per-key across three inits; init_server() still throws on a consumed server) — suite: 11 passed, 1 pre-existing skip.
• multi_panel/close_panel.spec.ts (exercises <regular-layout>/<regular-layout-frame> heavily): 6 passed.
• End-to-end: importing the rebuilt perspective-anywidget.js twice via fresh blob URLs (mimicking two anywidget models) — both evaluations succeed, both exported ready promises resolve against the same runtime, zero console errors. Previously the second import threw with NotSupportedError: the name "regular-layout" has already been used with this registry.

[texodus]

Thanks for the PR!

The await import makes the viewer ESM dist an async module (top-level await).

I've updated regular-layout texodus/regular-layout#20 and reverted this section 2739e9e

Semantics worth a look: re-registering an unconsumed key still overwrites its source

I don't know what the semantics of this should be. I'd prefer error, maybe with a "Reconsider your choices" message, except it seems many popular platforms do this. I did not realize Anywidget re-evaluates the module per model ... this library is ~400k with charts and datagrid that loads, parses, then subsequently ignored and leaks (I assume). I will do a follow-up that memoizes the bulk of Perspective's application code for anywidget to prevent this.