fix: prevent Nav flickering on mobile (#164)

Firefox for Android occasionally emits multiple scroll events with the
same `scrollTop` integer value towards the end of an inertia scroll,
which causes the Nav to flicker between visible states. This fixes it by
keeping the visible state the same if `scrollTop` is unchanged.

Author: zqianem

.changeset/yellow-islands-pump.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/site-kit': patch
+---
+
+Fix nav flickering on mobile

packages/site-kit/src/lib/nav/Nav.svelte

@@ -37,7 +37,9 @@ Top navigation bar for the application. It provides a slot for the left side, th
 
 		const scroll = root_scroll_element.scrollTop;
 		if (!hash_changed) {
-			visible = scroll < 50 || scroll < last_scroll;
+			visible = scroll === last_scroll
+				? visible
+				: scroll < 50 || scroll < last_scroll;
 		}
 
 		last_scroll = scroll;