feat: simpler hydration of CSS custom property wrappers

.changeset/funny-dragons-double.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: simpler hydration of CSS custom property wrappers

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -896,30 +896,35 @@ function serialize_inline_component(node, component_name, context) {
 					'$.spread_props',
 					...props_and_spreads.map((p) => (Array.isArray(p) ? b.object(p) : p))
 				);
-	/** @param {import('estree').Identifier} node_id */
-	let fn = (node_id) =>
-		b.call(
+
+	/** @param {import('estree').Expression} node_id */
+	let fn = (node_id) => {
+		return b.call(
 			context.state.options.dev
 				? b.call('$.validate_component', b.id(component_name))
 				: component_name,
 			node_id,
 			props_expression
 		);
+	};
 
 	if (bind_this !== null) {
 		const prev = fn;
-		fn = (node_id) =>
-			serialize_bind_this(
+
+		fn = (node_id) => {
+			return serialize_bind_this(
 				/** @type {import('estree').Identifier | import('estree').MemberExpression} */ (bind_this),
 				context,
 				prev(node_id)
 			);
+		};
 	}
 
 	if (node.type === 'SvelteComponent') {
 		const prev = fn;
+
 		fn = (node_id) => {
-			let component = b.call(
+			return b.call(
 				'$.component',
 				b.thunk(/** @type {import('estree').Expression} */ (context.visit(node.expression))),
 				b.arrow(
@@ -933,31 +938,26 @@ function serialize_inline_component(node, component_name, context) {
 					])
 				)
 			);
-			return component;
 		};
 	}
 
+	const statements = [...snippet_declarations, ...binding_initializers];
+
 	if (Object.keys(custom_css_props).length > 0) {
-		const prev = fn;
-		fn = (node_id) =>
-			b.call(
-				'$.css_props',
-				node_id,
-				// TODO would be great to do this at runtime instead. Svelte 4 also can't handle cases today
-				// where it's not statically determinable whether the component is used in a svg or html context
-				context.state.metadata.namespace === 'svg' || context.state.metadata.namespace === 'mathml'
-					? b.false
-					: b.true,
-				b.thunk(b.object(custom_css_props)),
-				b.arrow([b.id('$$node')], prev(b.id('$$node')))
-			);
-	}
+		context.state.template.push(
+			context.state.metadata.namespace === 'svg'
+				? '<g><!></g>'
+				: '<div style="display: contents"><!></div>'
+		);
 
-	const statements = [
-		...snippet_declarations,
-		...binding_initializers,
-		b.stmt(fn(context.state.node))
-	];
+		statements.push(
+			b.stmt(b.call('$.css_props', context.state.node, b.thunk(b.object(custom_css_props)))),
+			b.stmt(fn(b.member(context.state.node, b.id('lastChild'))))
+		);
+	} else {
+		context.state.template.push('<!>');
+		statements.push(b.stmt(fn(context.state.node)));
+	}
 
 	return statements.length > 1 ? b.block(statements) : statements[0];
 }
@@ -2947,8 +2947,6 @@ export const template_visitors = {
 		}
 	},
 	Component(node, context) {
-		context.state.template.push('<!>');
-
 		const binding = context.state.scope.get(
 			node.name.includes('.') ? node.name.slice(0, node.name.indexOf('.')) : node.name
 		);
@@ -2974,13 +2972,10 @@ export const template_visitors = {
 		context.state.init.push(component);
 	},
 	SvelteSelf(node, context) {
-		context.state.template.push('<!>');
 		const component = serialize_inline_component(node, context.state.analysis.name, context);
 		context.state.init.push(component);
 	},
 	SvelteComponent(node, context) {
-		context.state.template.push('<!>');
-
 		let component = serialize_inline_component(node, '$$component', context);
 
 		context.state.init.push(component);

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -907,7 +907,6 @@ function serialize_element_spread_attributes(
  * @param {import('#compiler').Component | import('#compiler').SvelteComponent | import('#compiler').SvelteSelf} node
  * @param {string | import('estree').Expression} component_name
  * @param {import('./types').ComponentContext} context
- * @returns {import('estree').Statement}
  */
 function serialize_inline_component(node, component_name, context) {
 	/** @type {Array<import('estree').Property[] | import('estree').Expression>} */
@@ -1103,6 +1102,10 @@ function serialize_inline_component(node, component_name, context) {
 		)
 	);
 
+	if (snippet_declarations.length > 0) {
+		statement = b.block([...snippet_declarations, statement]);
+	}
+
 	if (custom_css_props.length > 0) {
 		statement = b.stmt(
 			b.call(
@@ -1113,13 +1116,13 @@ function serialize_inline_component(node, component_name, context) {
 				b.thunk(b.block([statement]))
 			)
 		);
-	}
 
-	if (snippet_declarations.length > 0) {
-		statement = b.block([...snippet_declarations, statement]);
+		context.state.template.push(t_statement(statement));
+	} else {
+		context.state.template.push(block_open);
+		context.state.template.push(t_statement(statement));
+		context.state.template.push(block_close);
 	}
-
-	return statement;
 }
 
 /**
@@ -1666,29 +1669,17 @@ const template_visitors = {
 		}
 	},
 	Component(node, context) {
-		const state = context.state;
-		state.template.push(block_open);
-		const call = serialize_inline_component(node, node.name, context);
-		state.template.push(t_statement(call));
-		state.template.push(block_close);
+		serialize_inline_component(node, node.name, context);
 	},
 	SvelteSelf(node, context) {
-		const state = context.state;
-		state.template.push(block_open);
-		const call = serialize_inline_component(node, context.state.analysis.name, context);
-		state.template.push(t_statement(call));
-		state.template.push(block_close);
+		serialize_inline_component(node, context.state.analysis.name, context);
 	},
 	SvelteComponent(node, context) {
-		const state = context.state;
-		state.template.push(block_open);
-		const call = serialize_inline_component(
+		serialize_inline_component(
 			node,
 			/** @type {import('estree').Expression} */ (context.visit(node.expression)),
 			context
 		);
-		state.template.push(t_statement(call));
-		state.template.push(block_close);
 	},
 	LetDirective(node, { state }) {
 		if (node.expression && node.expression.type !== 'Identifier') {

packages/svelte/src/internal/client/dom/blocks/css-props.js

@@ -1,64 +1,35 @@
-import { namespace_svg } from '../../../../constants.js';
-import { hydrate_anchor, hydrate_start, hydrating } from '../hydration.js';
-import { empty } from '../operations.js';
+import { hydrating, set_hydrate_nodes } from '../hydration.js';
 import { render_effect } from '../../reactivity/effects.js';
 
 /**
- * @param {Element | Text | Comment} anchor
- * @param {boolean} is_html
- * @param {() => Record<string, string>} props
- * @param {(anchor: Element | Text | Comment) => any} component
+ * @param {HTMLDivElement | SVGGElement} element
+ * @param {() => Record<string, string>} get_styles
  * @returns {void}
  */
-export function css_props(anchor, is_html, props, component) {
-	/** @type {HTMLElement | SVGElement} */
-	let element;
-
-	/** @type {Text | Comment} */
-	let component_anchor;
-
+export function css_props(element, get_styles) {
 	if (hydrating) {
-		// Hydration: css props element is surrounded by a ssr comment ...
-		element = /** @type {HTMLElement | SVGElement} */ (hydrate_start);
-		// ... and the child(ren) of the css props element is also surround by a ssr comment
-		component_anchor = /** @type {Comment} */ (
-			hydrate_anchor(/** @type {Comment} */ (element.firstChild))
+		set_hydrate_nodes(
+			/** @type {import('#client').TemplateNode[]} */ ([...element.childNodes]).slice(0, -1)
 		);
-	} else {
-		if (is_html) {
-			element = document.createElement('div');
-			element.style.display = 'contents';
-		} else {
-			element = document.createElementNS(namespace_svg, 'g');
-		}
-
-		anchor.before(element);
-		component_anchor = element.appendChild(empty());
 	}
 
-	component(component_anchor);
-
 	render_effect(() => {
-		/** @type {Record<string, string>} */
-		let current_props = {};
-
 		render_effect(() => {
-			const next_props = props();
+			var styles = get_styles();
 
-			for (const key in current_props) {
-				if (!(key in next_props)) {
+			for (var key in styles) {
+				var value = styles[key];
+
+				if (value) {
+					element.style.setProperty(key, value);
+				} else {
 					element.style.removeProperty(key);
 				}
 			}
-
-			for (const key in next_props) {
-				element.style.setProperty(key, next_props[key]);
-			}
-
-			current_props = next_props;
 		});
 
 		return () => {
+			// TODO use `teardown` instead of creating a nested effect, post-https://github.com/sveltejs/svelte/pull/11936
 			element.remove();
 		};
 	});

packages/svelte/src/internal/server/index.js

@@ -162,15 +162,15 @@ export function attr(name, value, boolean) {
 export function css_props(payload, is_html, props, component) {
 	const styles = style_object_to_string(props);
 	if (is_html) {
-		payload.out += `<div style="display: contents; ${styles}"><!--[-->`;
+		payload.out += `<div style="display: contents; ${styles}">`;
 	} else {
-		payload.out += `<g style="${styles}"><!--[-->`;
+		payload.out += `<g style="${styles}">`;
 	}
 	component();
 	if (is_html) {
-		payload.out += `<!--]--></div>`;
+		payload.out += `<!----></div>`;
 	} else {
-		payload.out += `<!--]--></g>`;
+		payload.out += `<!----></g>`;
 	}
 }