fix: ensure that CSS is generated for the final frame of a transition (#11251)

* fix: ensure that CSS is generated for the final frame of a transition

* well that makes a lot more sense

Author: Rich-Harris

.changeset/funny-ties-jump.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure that CSS is generated for the final frame of a transition

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

@@ -322,7 +322,7 @@ function animate(element, options, counterpart, t2, callback) {
 	if (css) {
 		// WAAPI
 		var keyframes = [];
-		var n = duration / (1000 / 60);
+		var n = Math.ceil(duration / (1000 / 60)); // `n` must be an integer, or we risk missing the `t2` value
 
 		for (var i = 0; i <= n; i += 1) {
 			var t = t1 + delta * easing(i / n);

packages/svelte/tests/runtime-legacy/samples/transition-css-iframe/_config.js

@@ -19,6 +19,6 @@ export default test({
 		assert.equal(div.style.opacity, '0.25');
 
 		raf.tick(35);
-		assert.equal(div.style.opacity, '0.18333333333333335');
+		assert.equal(div.style.opacity, '0.15');
 	}
 });