fix: transitions might render animated elements without animation styles applied for the duration of some rendering frames when they start (#16035)

* fix: transitions might render animated elements without animation styles applied for the duration of some rendering frames when they starts

* add changeset

* Apply suggestions from code review

---------

Co-authored-by: Rich Harris <hello@rich-harris.dev>

.changeset/tiny-poems-scream.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: use `fill: 'forwards'` on transition animations to prevent flicker

packages/svelte/src/internal/client/dom/elements/transitions.js

@@ -381,9 +381,15 @@ function animate(element, options, counterpart, t2, on_finish) {
 	// create a dummy animation that lasts as long as the delay (but with whatever devtools
 	// multiplier is in effect). in the common case that it is `0`, we keep it anyway so that
 	// the CSS keyframes aren't created until the DOM is updated
-	var animation = element.animate(keyframes, { duration: delay });
+	//
+	// fill forwards to prevent the element from rendering without styles applied
+	// see https://github.com/sveltejs/svelte/issues/14732
+	var animation = element.animate(keyframes, { duration: delay, fill: 'forwards' });
 
 	animation.onfinish = () => {
+		// remove dummy animation from the stack to prevent conflict with main animation
+		animation.cancel();
+
 		// for bidirectional transitions, we start from the current position,
 		// rather than doing a full intro/outro
 		var t1 = counterpart?.t() ?? 1 - t2;