Svelte 5's mount() does not return prop accessors — setting
'instance.items = newValue' is a no-op. Switching to a single $state
proxy passed as 'model' lets the parent mutate fields and have the
dropdown react. The prop is named 'model' (not 'state') because the
$state rune name shadows a 'state' identifier in Svelte 5 templates.
Position class also switches from absolute to fixed so viewport-
relative DOMRect coordinates from clientRect() work when the dropdown
is mounted on document.body.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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