feat: render broken-link placeholder per element in linksToMany

[habdelra]

Preview

A linksToMany of three pets where the middle reference is broken (synthetic 404). Each view format (fitted / embedded / atom) and the edit editor render the broken element as a per-element placeholder while the present siblings render normally. Synthetic URLs only.

Summary

The plural linksToMany view path and both link editors (standard + compact) now read getRelationship once per render and dispatch BrokenLinkTemplate per slot whose relationship is error or not-found. Sibling states keep their existing render paths:

• error / not-found → broken-link placeholder in that slot, carrying the broken URL and error doc.
• present → the linked card renders normally.
• not-loaded / not-set → unchanged.

Each placeholder occupies its own .linksToMany-itemContainer, so a single broken element no longer breaks the surrounding list. The view-path placeholder adopts the list's effective format (mapped through brokenLinkFormat); the editors render it at the item footprint (fitted standard / atom compact) alongside the existing remove affordance, so a dead reference is visible and clearable rather than rendering as a broken card.

Stability

This field is sensitive to component stability — an unnecessary re-render in edit format can steal focus from an input as the user types. The change is built to avoid that:

• The {{#each}} keying is left untouched: the view path still keys on the stable per-child component identity, the editors on their index / box identity. The broken-link state only feeds an inner {{#if}} branch.
• getRelationship is a pure read (it never retriggers lazilyLoadLink) and is read once per render via {{#let}}; its result is a fresh array that is never used as an each key.

Net effect: per-slot identity stays stable when state hasn't changed, so a broken neighbor never tears down a focused sibling.

BrokenLinkTemplate now accepts splattributes so callers can attach per-element hooks/attributes to the placeholder root.

Stacked on #4979 (singular linksTo placeholder), reusing its BrokenLinkTemplate dispatch + brokenLinkFormat plumbing. Base retargets to main once the parent merges.

Ticket: CS-11226

Test plan

New tests/integration/components/linksto-many-broken-link-placeholder-test.gts (7/7 passing locally):

• a mixed list (present / not-found / present) renders each element correctly — the broken slot shows the placeholder in place while both healthy siblings render their cards
• a link-error sentinel element renders the error placeholder (fitted footprint) with its broken URL
• a list of only present links renders no placeholder
• the placeholder occupies the same per-element item container (fitted footprint) as a card would
• a list with one broken element converges in ≤2 renders (initial + post-lazy-load)
• in edit format a broken element shows the placeholder + remove affordance, and removing it leaves the healthy sibling untouched
• editing a sibling field while a broken element is present does not tear down the present element (DOM identity preserved across the re-render) — the focus-stability guard

Regression: the full links-filtered host suite is green locally (221/221), covering operator-mode | links, the linksTo/linksToMany card-basics and serialization paths, and the singular broken-link placeholder suite. Host lint:types (glint) and base hbs lint are clean.

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

Host Test Results

    1 files      1 suites   1h 46m 15s ⏱️
2 813 tests 2 798 ✅ 15 💤 0 ❌
2 832 runs  2 817 ✅ 15 💤 0 ❌

Results for commit 23004c5.

Realm Server Test Results

    1 files      1 suites   9m 50s ⏱️
1 502 tests 1 502 ✅ 0 💤 0 ❌
1 593 runs  1 593 ✅ 0 💤 0 ❌

Results for commit 23004c5.

[Copilot]

Pull request overview

Extends the per-element broken-link placeholder pattern (previously added for singular linksTo in #4979) to plural linksToMany fields. Each broken slot in a linksToMany list now renders BrokenLinkTemplate in its own item container while present siblings render normally, in both view and edit (standard + compact) paths.

Changes:

• Add brokenSlotsFor helper and per-slot broken-link rendering to the linksToMany view path, standard editor, and compact editor — keying {{#each}} on existing stable identities to preserve focus stability.
• Export brokenLinkFormat from card-api and add splattributes (Element: HTMLDivElement, ...attributes) to BrokenLinkTemplate.
• Add integration test coverage for mixed lists, link-error sentinel, fitted footprint, render-count convergence, edit removal, and DOM identity stability across unrelated re-renders.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/base/links-to-many-component.gts	Per-slot broken-link dispatch in view path + standard/compact editors, plus brokenSlotsFor helper.
packages/base/default-templates/broken-link-template.gts	Accept splattributes on the root div for per-element hooks.
packages/base/card-api.gts	Export brokenLinkFormat so the plural component can reuse the format mapping.
packages/host/tests/integration/components/linksto-many-broken-link-placeholder-test.gts	New integration tests covering plural broken-link rendering, removal, and render stability.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.