Svelte $: blocks (microtasks) execute before setTimeout(1) macrotasks, breaking PostHog's loaded() → deferred pageview pattern. Fix: capture_pageview: 'history_change' + manual posthog.capture('$pageview') after register().
When integrating PostHog (or any SDK that defers work via setTimeout) in a SvelteKit app, Svelte's reactive $: blocks fire as microtasks after onMount returns. This means they execute BEFORE any setTimeout(…, 1) macrotask the SDK queued during init.
Concrete example: PostHog's _loaded() method calls loaded() callback synchronously, then defers the initial $pageview capture via setTimeout(() => _captureInitialPageview(), 1). If a Svelte reactive block calls posthog.reset() (e.g. for anonymous users), the reset runs as a microtask BEFORE the setTimeout fires, wiping registered super properties before the pageview captures them.
The loaded() callback approach (recommended by PostHog docs) does NOT solve this because the callback runs synchronously during _loaded(), but the pageview still defers to setTimeout(1) which loses to the Svelte microtask.
Diagnosis: Use before_send config hook to inspect actual event properties — get_property() after page load shows stale state (post-reset), not what the event captured.
Fix: Use capture_pageview: 'history_change' (disables auto initial pageview, keeps SPA tracking via pushState/popState), then manually call posthog.register({...}) followed by posthog.capture('$pageview'). The manual capture bakes properties into the event data synchronously — immutable once captured, so a later reset() cannot strip them.
Key source references (posthog-js packages/browser/src/posthog-core.ts):
_loaded() L1042: loaded() callback fires synchronously_loaded() L1060-1067: auto-pageview deferred via setTimeout(…, 1)_captureInitialPageview() L4111: uses { send_instantly: true } but properties are already baked in at calculateEventProperties() timeHistoryAutocapture.isEnabled (L31): checks config.capture_pageview === 'history_change'This pattern applies to any framework with synchronous reactive updates (Svelte, SolidJS, fine-grained reactivity systems) integrating SDKs that defer initialization steps via setTimeout.