fix: avoid NaN keyframe values in slide transition (#18430)

Fixes #14205

When a `slide` transition runs on an element without a layout box, for example inside a `display: none` parent, `getComputedStyle` returns values like `auto` for the animated dimensions. These were passed through `parseFloat` unguarded, so the generated css contained `height: NaNpx` and the browser rejected the keyframe property with an "Invalid keyframe value" warning on every transition.

Fix it by omitting properties if they have unparseable numbers - it's the same outcome but you don't see a warning.

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/tough-pandas-shout.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid `NaN` keyframe values in `slide` transition for elements without a layout box

packages/svelte/src/transition/index.js

@@ -28,6 +28,17 @@ function split_css_unit(value) {
 	return split ? [parseFloat(split[1]), split[2] || 'px'] : [/** @type {number} */ (value), 'px'];
 }
 
+/**
+ * Omits unresolved dimensions rather than passing invalid values to Web Animations.
+ *
+ * @param {string} property
+ * @param {number} value
+ * @param {number} t
+ */
+function css_dimension(property, value, t) {
+	return Number.isNaN(value) ? '' : `${property}: ${t * value}px;`;
+}
+
 /**
  * Animates a `blur` filter alongside an element's opacity.
  *
@@ -138,13 +149,13 @@ export function slide(node, { delay = 0, duration = 400, easing = cubic_out, axi
 		css: (t) =>
 			'overflow: hidden;' +
 			`opacity: ${Math.min(t * 20, 1) * opacity};` +
-			`${primary_property}: ${t * primary_property_value}px;` +
-			`padding-${secondary_properties[0]}: ${t * padding_start_value}px;` +
-			`padding-${secondary_properties[1]}: ${t * padding_end_value}px;` +
-			`margin-${secondary_properties[0]}: ${t * margin_start_value}px;` +
-			`margin-${secondary_properties[1]}: ${t * margin_end_value}px;` +
-			`border-${secondary_properties[0]}-width: ${t * border_width_start_value}px;` +
-			`border-${secondary_properties[1]}-width: ${t * border_width_end_value}px;` +
+			css_dimension(primary_property, primary_property_value, t) +
+			css_dimension(`padding-${secondary_properties[0]}`, padding_start_value, t) +
+			css_dimension(`padding-${secondary_properties[1]}`, padding_end_value, t) +
+			css_dimension(`margin-${secondary_properties[0]}`, margin_start_value, t) +
+			css_dimension(`margin-${secondary_properties[1]}`, margin_end_value, t) +
+			css_dimension(`border-${secondary_properties[0]}-width`, border_width_start_value, t) +
+			css_dimension(`border-${secondary_properties[1]}-width`, border_width_end_value, t) +
 			`min-${primary_property}: 0`
 	};
 }

packages/svelte/tests/runtime-browser/samples/transition-slide-hidden-parent/_config.js

@@ -0,0 +1,27 @@
+import { ok, test } from '../../assert';
+
+export default test({
+	async test({ assert, window }) {
+		window.document.querySelector('button')?.click();
+		await new Promise((r) => setTimeout(r, 100));
+
+		const p = window.document.querySelector('p');
+		const animations = /** @type {HTMLElement} */ (p).getAnimations();
+		assert.equal(animations.length, 1);
+
+		// when the element has no layout box, computed dimensions resolve to 'auto',
+		// which must not end up as NaN values that the browser rejects (#14205)
+		const effect = /** @type {KeyframeEffect} */ (animations[0].effect);
+		const keyframes = effect.getKeyframes();
+		ok(keyframes.length > 0);
+		assert.equal(effect.getTiming().duration, 400);
+
+		for (const keyframe of keyframes) {
+			ok(!('height' in keyframe), 'unresolved height should be omitted');
+
+			for (const value of Object.values(keyframe)) {
+				ok(!String(value).includes('NaN'), `unexpected NaN in keyframe: ${value}`);
+			}
+		}
+	}
+});

packages/svelte/tests/runtime-browser/samples/transition-slide-hidden-parent/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	import { slide } from 'svelte/transition';
+
+	let visible = $state(false);
+</script>
+
+<button onclick={() => (visible = !visible)}>toggle</button>
+
+<div style="display: none">
+	{#if visible}
+		<p transition:slide>hello</p>
+	{/if}
+</div>