fix: race condition in `svelte:element` with transition #7948 (#7949)

fixes #7948
- The assignment of the variable "previous_tag" was incorrectly positioned and could cause race condition when used with transitions.
- We need another variable to detect when we are in a transition to remove a node

---------

Co-authored-by: Yuichiro Yamashita <xydybaseball@gmail.com>

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -319,10 +319,16 @@ export default class ElementWrapper extends Wrapper {
 		const has_transitions = !!(this.node.intro || this.node.outro);
 		const not_equal = this.renderer.component.component_options.immutable ? x`@not_equal` : x`@safe_not_equal`;
 
+		const tag_will_be_removed = block.get_unique_name('tag_will_be_removed');
+		if (has_transitions) {
+			block.add_variable(tag_will_be_removed, x`false`);
+		}
+
 		block.chunks.update.push(b`
 			if (${tag}) {
 				if (!${previous_tag}) {
 					${this.var} = ${this.child_dynamic_element_block.name}(#ctx);
+					${previous_tag} = ${tag};
 					${this.var}.c();
 					${has_transitions && b`@transition_in(${this.var})`}
 					${this.var}.m(${this.get_update_mount_node(anchor)}, ${anchor});
@@ -331,27 +337,39 @@ export default class ElementWrapper extends Wrapper {
 					${this.renderer.options.dev && b`@validate_dynamic_element(${tag});`}
 					${this.renderer.options.dev && this.node.children.length > 0 && b`@validate_void_dynamic_element(${tag});`}
 					${this.var} = ${this.child_dynamic_element_block.name}(#ctx);
+					${previous_tag} = ${tag};
 					${this.var}.c();
+					${has_transitions && b`if (${tag_will_be_removed}) {
+						${tag_will_be_removed} = false;
+						@transition_in(${this.var})
+					}`}
 					${this.var}.m(${this.get_update_mount_node(anchor)}, ${anchor});
 				} else {
+					${has_transitions && b`if (${tag_will_be_removed}) {
+						${tag_will_be_removed} = false;
+						@transition_in(${this.var})
+					}`}
 					${this.var}.p(#ctx, #dirty);
 				}
 			} else if (${previous_tag}) {
 				${has_transitions
 				? b`
+							${tag_will_be_removed} = true;
 							@group_outros();
 							@transition_out(${this.var}, 1, 1, () => {
 								${this.var} = null;
+								${previous_tag} = ${tag};
+								${tag_will_be_removed} = false;
 							});
 							@check_outros();
 						`
 				: b`
 							${this.var}.d(1);
 							${this.var} = null;
+							${previous_tag} = ${tag};
 						`
 			}
 			}
-			${previous_tag} = ${tag};
 		`);
 
 		if (this.child_dynamic_element_block.has_intros) {

test/js/samples/svelte-element-event-handlers/expected.js

@@ -112,11 +112,13 @@ function create_fragment(ctx) {
 			if (a) {
 				if (!previous_tag) {
 					svelte_element = create_dynamic_element(ctx);
+					previous_tag = a;
 					svelte_element.c();
 					svelte_element.m(svelte_element_anchor.parentNode, svelte_element_anchor);
 				} else if (safe_not_equal(previous_tag, a)) {
 					svelte_element.d(1);
 					svelte_element = create_dynamic_element(ctx);
+					previous_tag = a;
 					svelte_element.c();
 					svelte_element.m(svelte_element_anchor.parentNode, svelte_element_anchor);
 				} else {
@@ -125,9 +127,8 @@ function create_fragment(ctx) {
 			} else if (previous_tag) {
 				svelte_element.d(1);
 				svelte_element = null;
+				previous_tag = a;
 			}
-
-			previous_tag = a;
 		},
 		i: noop,
 		o: noop,

test/js/samples/svelte-element-svg/expected.js

@@ -72,11 +72,13 @@ function create_fragment(ctx) {
 			if (/*tag*/ ctx[0].svg) {
 				if (!previous_tag) {
 					svelte_element = create_dynamic_element(ctx);
+					previous_tag = /*tag*/ ctx[0].svg;
 					svelte_element.c();
 					svelte_element.m(svelte_element_anchor.parentNode, svelte_element_anchor);
 				} else if (safe_not_equal(previous_tag, /*tag*/ ctx[0].svg)) {
 					svelte_element.d(1);
 					svelte_element = create_dynamic_element(ctx);
+					previous_tag = /*tag*/ ctx[0].svg;
 					svelte_element.c();
 					svelte_element.m(svelte_element_anchor.parentNode, svelte_element_anchor);
 				} else {
@@ -85,9 +87,8 @@ function create_fragment(ctx) {
 			} else if (previous_tag) {
 				svelte_element.d(1);
 				svelte_element = null;
+				previous_tag = /*tag*/ ctx[0].svg;
 			}
-
-			previous_tag = /*tag*/ ctx[0].svg;
 		},
 		i: noop,
 		o: noop,

test/runtime/samples/component-transition/_config.js

@@ -0,0 +1,19 @@
+export default {
+	async test({ assert, target, raf }) {
+		const button = target.querySelector('#button');
+		const container = target.querySelector('#container');
+
+		// Multiple click on button
+		await button.dispatchEvent(new window.MouseEvent('click'));
+		await button.dispatchEvent(new window.MouseEvent('click'));
+		await button.dispatchEvent(new window.MouseEvent('click'));
+		await button.dispatchEvent(new window.MouseEvent('click'));
+		await button.dispatchEvent(new window.MouseEvent('click'));
+		await button.dispatchEvent(new window.MouseEvent('click'));
+		await button.dispatchEvent(new window.MouseEvent('click'));
+
+		assert.equal(container.children.length, 1);
+		raf.tick(501);
+		assert.equal(container.children.length, 0);
+	}
+};

test/runtime/samples/component-transition/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	import { slide } from 'svelte/transition';
+	let tag = 'div';
+	function toggle() {
+		tag = (tag) ? null : 'div';
+	}
+</script>
+
+<button id="button" on:click={toggle}>toggle</button> TAG={tag}
+
+<div id="container">
+	<svelte:element this={tag} transition:slide={{duration:500}}>CONTENT</svelte:element>	
+</div>