meta pixel: default to disablePushState = true

Breaks SvelteKit SPA which doesn't allow use of history API, requiring
its own wrapper to be used instead.

src/lib/types/fbq.d.ts

@@ -340,6 +340,11 @@ export interface FBQ {
 	// consent and LDU
 	(cmd: 'consent', state: 'grant' | 'revoke'): void;
 	(cmd: 'dataProcessingOptions', options: string[], countryCode?: number, stateCode?: number): void;
+
+	/** Prevent automatic listening to history.pushState/popstate */
+	disablePushState?: boolean;
+	/** Allow duplicate page view events (legacy / undocumented behavior) */
+	allowDuplicatePageViews?: boolean;
 }
 
 declare global {

src/lib/util/meta-pixel-loader.ts

@@ -9,22 +9,48 @@ type QueuedFBQ = ((...args: unknown[]) => void) & {
 	loaded?: boolean;
 	version?: string;
 	push?: unknown;
+	disablePushState?: boolean;
+	allowDuplicatePageViews?: boolean;
 };
 
 /**
  * Loads the Meta Pixel script and configures the `fbq` function to queue
  * commands until the script is fully loaded. You may optionally await the
  * returned Promise to ensure the script has loaded before proceeding.
+ *
+ * Options:
+ * - `disablePushState` (default: true) — when true, sets
+ *   `window.fbq.disablePushState = true` before the pixel script loads so the
+ *   pixel does not auto-listen to `history.pushState`/`popstate` (recommended
+ *   for SPA frameworks like Svelte).
+ * - `allowDuplicatePageViews` (default: false) — when true, sets
+ *   `window.fbq.allowDuplicatePageViews = true` on the stub.
  */
-export const loadMetaPixel = (): Promise<void> => {
+export const loadMetaPixel = (opts?: {
+	disablePushState?: boolean;
+	allowDuplicatePageViews?: boolean;
+}): Promise<void> => {
 	// Make sure we're using the browser
 	if (!browser || !window) {
 		return Promise.reject(new Error(`Not in browser, can't access window`));
 	}
 
+	// Default behavior: disable pushState handling since Svelte apps manage
+	// navigation themselves and Meta's auto-patching of history APIs can
+	// cause duplicate/incorrect pageview events. Consumers can pass
+	// `opts.disablePushState = false` to opt out.
+	const disablePushState = opts?.disablePushState ?? true;
+	const allowDuplicatePageViews = opts?.allowDuplicatePageViews ?? false;
+
 	// If fbq is already defined, resolve immediately
 	const existing = window.fbq as QueuedFBQ | undefined;
 	if (existing) {
+		// If the existing stub is present but hasn't set these flags yet, set
+		// them now so the loaded library (if it inspects them) sees intended
+		// behavior. Setting these is a no-op if initialization already
+		// completed.
+		if (disablePushState) existing.disablePushState = true;
+		if (allowDuplicatePageViews) existing.allowDuplicatePageViews = true;
 		const existingScript = getExistingScript();
 		if (existingScript) {
 			return new Promise((resolve, reject) => {
@@ -53,6 +79,9 @@ export const loadMetaPixel = (): Promise<void> => {
 	q.push = q;
 	q.loaded = true;
 	q.version = '2.0';
+	// set control flags on the stub before the meta script runs
+	if (disablePushState) q.disablePushState = true;
+	if (allowDuplicatePageViews) q.allowDuplicatePageViews = true;
 	window.fbq = q;
 	window._fbq = q;