From faf1e103c53675cbe71860f331d4fedd84c8d612 Mon Sep 17 00:00:00 2001
From: Siddhesh Kabra <146343711+Xsidz@users.noreply.github.com>
Date: Fri, 21 Aug 2026 02:20:44 +0530
Subject: [PATCH] fix: prevent onoutroend from firing twice when
 compilerOptions.hmr is true (#18655)

Fixes #18440

`hmr()` in `packages/svelte/src/internal/client/dev/hmr.js` wrapped a
component in a `block` effect containing a `branch` effect. It forwarded
the inner effect's `nodes` object to the outer block:

This shared the same reference, meaning `outer_block.nodes ===
inner_branch.nodes`. When `pause_children` collected transitions for
unmounting, Each collected transition had `out(check)` called multiple times,
starting multiple animations and firing `outroend` multiple times.

Fix by only copying `start`/`end` (the DOM range info the outer block needs)
without sharing the transitions array:
---
 .changeset/fix-hmr-onoutroend-double-fire.md  |  5 +++++
 .../svelte/src/internal/client/dev/hmr.js     | 19 +++++++++++++++----
 2 files changed, 20 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/fix-hmr-onoutroend-double-fire.md

diff --git a/.changeset/fix-hmr-onoutroend-double-fire.md b/.changeset/fix-hmr-onoutroend-double-fire.md
new file mode 100644
index 0000000000..0eafbfa731
--- /dev/null
+++ b/.changeset/fix-hmr-onoutroend-double-fire.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent onoutroend from firing twice when compilerOptions.hmr is true
diff --git a/packages/svelte/src/internal/client/dev/hmr.js b/packages/svelte/src/internal/client/dev/hmr.js
index 73dba95f9b..0e988fe66d 100644
--- a/packages/svelte/src/internal/client/dev/hmr.js
+++ b/packages/svelte/src/internal/client/dev/hmr.js
@@ -57,10 +57,21 @@ export function hmr(fn) {
 				if (ran) set_should_intro(true);
 			});
 
-			// Forward the nodes from the inner effect to the outer active effect which would
-			// get them if the HMR wrapper wasn't there. Do this inside the block not outside
-			// so that HMR updates to the component will also update the nodes on the active effect.
-			/** @type {Effect} */ (active_effect).nodes = effect.nodes;
+			// Forward the start/end DOM nodes from the inner effect to the outer active effect
+			// which would get them if the HMR wrapper wasn't there. Do this inside the block not
+			// outside so that HMR updates to the component will also update the nodes on the
+			// active effect. We copy only start/end, not the full nodes object, so that
+			// pause_children does not collect transitions from both effects and fire outroend twice.
+			var inner_nodes = effect.nodes;
+			if (inner_nodes) {
+				var ae = /** @type {Effect} */ (active_effect);
+				if (ae.nodes) {
+					ae.nodes.start = inner_nodes.start;
+					ae.nodes.end = inner_nodes.end;
+				} else {
+					ae.nodes = { start: inner_nodes.start, end: inner_nodes.end, a: null, t: null };
+				}
+			}
 		}, EFFECT_TRANSPARENT);
 
 		ran = true;