Port the rest of the relate-picker's interactive behaviours from Playwright into
the SX harness, driving the real engine against the mock DOM:
- load: the form's "load" trigger populates the results on first render
- filter: a debounced "input" re-fetches and narrows the candidates
- paging: revealing the load-more sentinel pages in the next page (outerHTML
swap replaces the sentinel)
- error-retry: a dropped fetch marks .sx-error, and the next request clears it
Models two browser natives the OCaml runner lacks: observe-intersection (a
recording stub the test fires to simulate the sentinel scrolling into view) and
the synchronous-timer retry (stripped in the error test — backoff timing is a
test-engine.sx concern; here we assert the visible state).
Mock-DOM completeness (run_tests.ml): firstChild/lastChild on elements, so
children-to-fragment can drain a parsed fragment into an innerHTML/outerHTML swap
target. (Also repairs one pre-existing web test that needed firstChild.)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
giles
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