fix: avoid NaN keyframe values in slide transition

.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 +28,18 @@ function split_css_unit(value) {
 	return split ? [parseFloat(split[1]), split[2] || 'px'] : [/** @type {number} */ (value), 'px'];
 }
 
+/**
+ * Elements without a layout box (e.g. inside a `display: none` parent) have computed
+ * dimensions like `auto` that parse to `NaN`, producing keyframe values the browser
+ * rejects (https://github.com/sveltejs/svelte/issues/14205). Treat them as `0` instead
+ * @param {string} value
+ * @returns {number}
+ */
+function parse_dimension(value) {
+	const result = parseFloat(value);
+	return Number.isNaN(result) ? 0 : result;
+}
+
 /**
  * Animates a `blur` filter alongside an element's opacity.
  *
@@ -116,19 +128,21 @@ export function slide(node, { delay = 0, duration = 400, easing = cubic_out, axi
 
 	const opacity = +style.opacity;
 	const primary_property = axis === 'y' ? 'height' : 'width';
-	const primary_property_value = parseFloat(style[primary_property]);
+	const primary_property_value = parse_dimension(style[primary_property]);
 	const secondary_properties = axis === 'y' ? ['top', 'bottom'] : ['left', 'right'];
 	const capitalized_secondary_properties = secondary_properties.map(
 		(e) => /** @type {'Left' | 'Right' | 'Top' | 'Bottom'} */ (`${e[0].toUpperCase()}${e.slice(1)}`)
 	);
-	const padding_start_value = parseFloat(style[`padding${capitalized_secondary_properties[0]}`]);
-	const padding_end_value = parseFloat(style[`padding${capitalized_secondary_properties[1]}`]);
-	const margin_start_value = parseFloat(style[`margin${capitalized_secondary_properties[0]}`]);
-	const margin_end_value = parseFloat(style[`margin${capitalized_secondary_properties[1]}`]);
-	const border_width_start_value = parseFloat(
+	const padding_start_value = parse_dimension(
+		style[`padding${capitalized_secondary_properties[0]}`]
+	);
+	const padding_end_value = parse_dimension(style[`padding${capitalized_secondary_properties[1]}`]);
+	const margin_start_value = parse_dimension(style[`margin${capitalized_secondary_properties[0]}`]);
+	const margin_end_value = parse_dimension(style[`margin${capitalized_secondary_properties[1]}`]);
+	const border_width_start_value = parse_dimension(
 		style[`border${capitalized_secondary_properties[0]}Width`]
 	);
-	const border_width_end_value = parseFloat(
+	const border_width_end_value = parse_dimension(
 		style[`border${capitalized_secondary_properties[1]}Width`]
 	);
 	return {

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

@@ -0,0 +1,25 @@
+import { 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 keyframes = /** @type {KeyframeEffect} */ (animations[0].effect).getKeyframes();
+		assert.ok(keyframes.length > 0);
+
+		for (const keyframe of keyframes) {
+			assert.ok('height' in keyframe, 'height was rejected as an invalid keyframe value');
+
+			for (const value of Object.values(keyframe)) {
+				assert.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>

packages/svelte/tests/runtime-legacy/samples/class-shortcut-with-transition/_config.js

@@ -18,7 +18,7 @@ export default test({
 		raf.tick(150);
 		assert.htmlEqual(
 			target.innerHTML,
-			'<p>foo</p><p class="red svelte-1yszte8 border" style="overflow: hidden; opacity: 0; border-top-width: 0.5px; border-bottom-width: 0.5px; min-height: 0;">bar</p>'
+			'<p>foo</p><p class="red svelte-1yszte8 border" style="overflow: hidden; opacity: 0; height: 0px; padding-top: 0px; padding-bottom: 0px; margin-top: 0px; margin-bottom: 0px; border-top-width: 0.5px; border-bottom-width: 0.5px; min-height: 0;">bar</p>'
 		);
 		component.open = true;
 		raf.tick(250);