Editor page canvas stuck on spinner in local dev: section iframes never render #1235

Summary

Opening a page in the editor under primo dev locally leaves the main canvas on a permanent loading spinner. Section wrappers are created in the DOM but each one is empty — ComponentNode never renders its <iframe>, so the page never reports itself mounted.

Reproduced on unmodified main (fd175fa / 6558bca). Not caused by any in-flight branch — I hit this while verifying #1233 and checked out pristine main to confirm.

Reproduce

1. cd primo-sites && primo dev (a workspace with a multi-section page — used coffee-shop, pages/about.yaml, 5 sections)
2. Open http://coffee-shop.localhost:3000/admin/site/about
3. Canvas spins indefinitely

Observed

// after 8s on /admin/site/about
{ sectionNodes: 5, iframes: 0, spinnerVisible: true, allWrappersEmpty: true }

All 5 [data-section] wrappers are present, each children.length === 0 and 32px tall. No console errors (only an unrelated __primo_dev_ws__ websocket warning). Data loads fine — every page_sections / site_symbols / page_section_entries request returns 200, and the records exist when queried with an auth token.

Why it hangs

The spinner is gated on a mount counter in src/lib/builder/views/editor/Page.svelte:

{#if !page_mounted && sections && sections.length > 1}

page_mounted only becomes true once sections_mounted >= sections.length. Each increment comes from on:mount={() => sections_mounted++} on ComponentNode, dispatched by dispatch_mount().

But every dispatch_mount() path depends on the iframe existing — onload={setup_component_iframe}, the MutationObserver, the component-error message handler — and the iframe itself is gated at ComponentNode.svelte:1262:

{#if $site_html && generated_js}
  <iframe ... onload={setup_component_iframe}></iframe>
{/if}

So when that condition stays false, no iframe → no mount → counter never reaches target → spinner forever. $site_html does get populated (the page <head> has 80 children), which points at generated_js never being produced — i.e. the per-block processCode compile in ComponentNode isn't completing or isn't assigning.

The actual bug

Independent of why the compile doesn't finish, the failure mode is wrong: a block that never compiles produces no iframe, no error branch, and no mount signal, so the whole canvas is held hostage by one silent block. There's an {#if error} branch, but the paths that set error also require the iframe to exist, so a pre-iframe compile failure can't reach it.

Worth fixing in two parts:

1. Root cause — why generated_js is never set for these blocks in local dev.
2. Resilience — the mount counter shouldn't be able to deadlock. A block that fails to compile should surface its error and still count as "settled", so one bad block degrades to a visible error instead of an indefinite spinner with no diagnostics.

Impact

The editor's primary view is unusable locally — you can still navigate sidebars, page types, modals and block code, but not the page canvas. Verification of #1233 had to route around it.

Environment

• main @ 6558bca (also repros at fd175fa)
• CLI 0.1.24, primo dev (both --author files and --author cms)
• macOS, Chromium