basic support for style:display|transition

packages/svelte/src/compiler/phases/2-analyze/visitors/StyleDirective.js

@@ -10,7 +10,12 @@ import { mark_subtree_dynamic } from './shared/fragment.js';
  */
 export function StyleDirective(node, context) {
 	if (node.modifiers.length > 1 || (node.modifiers.length && node.modifiers[0] !== 'important')) {
-		e.style_directive_invalid_modifier(node);
+		if (
+			node.name !== 'display' ||
+			node.modifiers.findIndex((m) => m !== 'important' && m !== 'transition') >= 0
+		) {
+			e.style_directive_invalid_modifier(node);
+		}
 	}
 
 	mark_subtree_dynamic(context.path);

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

@@ -148,7 +148,11 @@ export function RegularElement(node, context) {
 				break;
 
 			case 'StyleDirective':
-				style_directives.push(attribute);
+				if (attribute.name === 'display' && attribute.modifiers.includes('transition')) {
+					style_display = attribute;
+				} else {
+					style_directives.push(attribute);
+				}
 				break;
 
 			case 'TransitionDirective':
@@ -167,6 +171,10 @@ export function RegularElement(node, context) {
 	}
 
 	if (display_directive) {
+		if (style_display !== null) {
+			// TODO
+			throw new Error('#display and style:display|transition forbidden');
+		}
 		// When we have a #display directive, the style:display directive must be handheld differently
 		const index = style_directives.findIndex((d) => d.name === 'display');
 		if (index >= 0) {
@@ -423,7 +431,11 @@ export function RegularElement(node, context) {
 		}
 	}
 
-	if (display_directive || node.fragment.nodes.some((node) => node.type === 'SnippetBlock')) {
+	if (
+		style_display ||
+		display_directive ||
+		node.fragment.nodes.some((node) => node.type === 'SnippetBlock')
+	) {
 		// Wrap children in `{...}` to avoid declaration conflicts
 		const block = b.block([
 			...child_state.init,
@@ -432,7 +444,7 @@ export function RegularElement(node, context) {
 			...child_state.after_update,
 			...element_state.after_update
 		]);
-		if (display_directive) {
+		if (display_directive || style_display) {
 			context.state.init.push(
 				build_display_directive(node_id, display_directive, style_display, block, context)
 			);

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

@@ -217,13 +217,15 @@ export function get_attribute_name(element, attribute) {
 
 /**
  * @param {Identifier} node_id
- * @param {AST.DisplayDirective} display
+ * @param {AST.DisplayDirective | null} display
  * @param {AST.StyleDirective | null} style
  * @param {BlockStatement} block
  * @param {ComponentContext} context
  */
 export function build_display_directive(node_id, display, style, block, context) {
-	const visibility = b.thunk(/** @type {Expression} */ (context.visit(display.expression)));
+	const visibility = display
+		? b.thunk(/** @type {Expression} */ (context.visit(display.expression)))
+		: b.literal(null);
 
 	/** @type {Expression | undefined} */
 	let value = undefined;

packages/svelte/src/compiler/types/template.d.ts

@@ -240,7 +240,7 @@ export namespace AST {
 		name: string;
 		/** The 'y' in `style:x={y}` */
 		value: true | ExpressionTag | Array<ExpressionTag | Text>;
-		modifiers: Array<'important'>;
+		modifiers: Array<'important' | 'transition'>;
 		/** @internal */
 		metadata: {
 			expression: ExpressionMetadata;

packages/svelte/src/internal/client/dom/blocks/display.js

@@ -7,7 +7,7 @@ import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
  * @param {HTMLElement} node
  * @param {() => any} get_visibility
  * @param {() => void} render_fn
- * @param {()=>string|null} [get_value]
+ * @param {()=>string|boolean|null} [get_value]
  * @param {boolean} [default_important]
  * @returns {void}
  */
@@ -41,8 +41,14 @@ export function display(node, get_visibility, render_fn, get_value, default_impo
 
 	const effect = block(render_fn);
 	template_effect(() => {
-		const visible = !!get_visibility();
-		const value = visible ? get_value?.() : 'none';
+		let display_value = get_value?.();
+		if (display_value === true) {
+			display_value = null;
+		} else if (display_value === false) {
+			display_value = 'none';
+		}
+		const visible = get_visibility !== null ? !!get_visibility() : display_value !== 'none';
+		const value = visible ? display_value : 'none';
 
 		if (visible === prev_visible && value === prev_value) {
 			return;