last fade attempt

src/lib/utils/url-state.ts

@@ -7,22 +7,31 @@
  * history entry or re-runs a load - the back button still means "the page
  * before", not "the previous day I clicked".
  *
- * Callers must pass the URL *untracked* (`get(page).url`, not `$page.url`).
- * Reading it reactively inside the same effect that writes it creates a loop:
- * replaceState publishes a new URL, the effect re-runs, writes again - which
- * Svelte eventually kills with `effect_update_depth_exceeded`, hanging the page.
+ * The base is `location`, deliberately not `page.url`. Shallow routing does not
+ * republish the URL: `replaceState` writes the history entry (and files the
+ * *previous* `page.url` in it, so a popstate can restore it) but leaves
+ * `page.url` on the last navigated URL. Diffing against that stale value is
+ * wrong in exactly one direction - clearing a parameter. Opening a day writes
+ * `?day=`, `page.url` still has none, so asking to remove it produces a URL
+ * identical to the stale one, the write is skipped as a no-op, and the
+ * parameter stays in the address bar for good.
+ *
+ * Reading `location` rather than a passed-in URL also removes the old trap that
+ * callers had to pass it untracked: an effect that both read `$page.url` and
+ * wrote to it looped until `effect_update_depth_exceeded` hung the page.
  */
 import { browser } from '$app/environment';
 import { replaceState } from '$app/navigation';
 
-export function syncSearchParams(url: URL, updates: Record<string, string | null>): void {
+export function syncSearchParams(updates: Record<string, string | null>): void {
 	if (!browser) return;
-	const next = new URL(url);
+	const current = new URL(window.location.href);
+	const next = new URL(current);
 	for (const [key, value] of Object.entries(updates)) {
 		if (value == null || value === '') next.searchParams.delete(key);
 		else next.searchParams.set(key, value);
 	}
-	if (next.href === url.href) return;
+	if (next.href === current.href) return;
 	try {
 		replaceState(next, {});
 	} catch {