fix: ensure each block animations don't mess with transitions (#17238)

This ensures that an animation does not run when the element is first transitioning in. Else the animation would mess with the transition (overriding its animation basically). Due to our test setup it's not testable but I veryfied it fixes #17181 (tested all reproductions in there)

.changeset/tiny-loops-wonder.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure each block animations don't mess with transitions

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -394,8 +394,12 @@ function reconcile(state, array, anchor, flags, get_key) {
 			key = get_key(value, i);
 			item = /** @type {EachItem} */ (items.get(key));
 
-			item.a?.measure();
-			(to_animate ??= new Set()).add(item);
+			// offscreen == coming in now, no animation in that case,
+			// else this would happen https://github.com/sveltejs/svelte/issues/17181
+			if (item.o) {
+				item.a?.measure();
+				(to_animate ??= new Set()).add(item);
+			}
 		}
 	}