Pass distinct props (testid) to each MockComponent instance

[thedavidmeister]

Fixes #1842

Motivation

The ui-components mock at packages/ui-components/src/lib/__mocks__/MockComponent.svelte still carried the reactive props store ($: $props = $$props) that #1840 identified as the cause of the RangeError: Invalid array length crash/hang when a test mounts more than one instance. PR #1841 only fixed the webapp copy of the mock; this one was left untouched. There was also no way to give each instance distinct props (e.g. a testid) so multiple instances could be told apart.

Solution

Rework the ui-components mock so it:

• Mirrors the received props into the shared props store from afterUpdate instead of a reactive $: block, so it never re-enters the component update cycle that exhausted the dirty array across multiple instances (the MockComponent is throwing invalid array length errors #1840 crash/hang).
• Exports a testid prop (default "mock-component") that drives data-testid, so a test mounting more than one instance can give each a distinct id and query them individually. Remaining props are still spread onto the element and into the store, so existing attribute- and store-based assertions keep working (License, CodeMirror*, TanstackLightweightChartLine).

Tests

Added MockComponent.test.ts (plus a MockComponent.test.svelte harness) with discriminating assertions:

• default data-testid is mock-component;
• a testid prop overrides data-testid and removes the default id;
• remaining props are spread as element attributes and testid is not duplicated as a testid attribute;
• the shared store mirrors received props (including non-serializable values like functions) and excludes testid;
• mounting more than one instance with distinct testids renders both, queryable individually, with per-instance props and slot content.

Verification

• Full ui-components vitest suite: 640 passed. The only red, ShareChoicesButton > shows and hides copied message when clicked, is a pre-existing flaky CSS-animation timing test unrelated to this change — it passes on an isolated rerun.
• Discrimination check: running the new MockComponent.test.ts against the original (unfixed) mock hangs on the multi-instance test — the exact MockComponent is throwing invalid array length errors #1840 symptom — confirming the test would catch a regression.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 31e7a49 and b294ced.

📒 Files selected for processing (3)

• packages/ui-components/src/__tests__/MockComponent.test.svelte
• packages/ui-components/src/__tests__/MockComponent.test.ts
• packages/ui-components/src/lib/__mocks__/MockComponent.svelte

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-13-issue-1842

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