From 4d6334d1fa1109bb7438a0f9c76bd9daac2202ae Mon Sep 17 00:00:00 2001
From: ENvironmentSet <sbshw1@gmail.com>
Date: Wed, 11 Mar 2026 22:39:56 +0900
Subject: [PATCH 1/2] fix(core): prevent duplicate setInterval in dispatchEvent
MIME-Version: 1.0
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When dispatchEvent was called rapidly (e.g., push → push → pop),
each call created a new setInterval without clearing the previous one.
This caused N concurrent intervals running aggregate + isEqual every
16ms, linearly increasing CPU load during transitions.

Fix by adding an `intervalRunning` flag so only one interval exists
at a time. Since all intervals read from the same `events.value`,
a single interval handles all queued events correctly.

Resolves FEP-1958

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 core/src/makeCoreStore.ts | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/core/src/makeCoreStore.ts b/core/src/makeCoreStore.ts
index cbdc99aaa..a892b1d2b 100644
--- a/core/src/makeCoreStore.ts
+++ b/core/src/makeCoreStore.ts
@@ -84,6 +84,8 @@ export function makeCoreStore(options: MakeCoreStoreOptions): CoreStore {
     value: aggregate(events.value, new Date().getTime()),
   };
 
+  let intervalRunning = false;
+
   const actions: StackflowActions = {
     getStack() {
       return stack.value;
@@ -98,6 +100,12 @@ export function makeCoreStore(options: MakeCoreStoreOptions): CoreStore {
       events.value.push(newEvent);
       setStackValue(nextStackValue);
 
+      if (intervalRunning) {
+        return;
+      }
+
+      intervalRunning = true;
+
       const interval = setInterval(() => {
         const nextStackValue = aggregate(events.value, new Date().getTime());
 
@@ -107,6 +115,7 @@ export function makeCoreStore(options: MakeCoreStoreOptions): CoreStore {
 
         if (nextStackValue.globalTransitionState === "idle") {
           clearInterval(interval);
+          intervalRunning = false;
         }
       }, INTERVAL_MS);
     },

From e723f021b2e76ec63650d1438069370b81252994 Mon Sep 17 00:00:00 2001
From: ENvironmentSet <sbshw1@gmail.com>
Date: Thu, 12 Mar 2026 16:07:55 +0900
Subject: [PATCH 2/2] fix(core): use currentInterval guard instead of
 intervalRunning flag

The intervalRunning flag caused a regression when onChanged hooks
triggered reentrant dispatchEvent calls: the nested dispatch would
skip interval creation, but the original interval would clear itself
on stale "idle" state, leaving no interval to poll the new transition.

Switch to clearInterval(currentInterval) + create new approach, which
safely handles reentrancy by always replacing the previous interval.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .changeset/fix-duplicate-interval.md |  5 +++++
 core/src/makeCoreStore.ts            | 13 +++++++------
 2 files changed, 12 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/fix-duplicate-interval.md

diff --git a/.changeset/fix-duplicate-interval.md b/.changeset/fix-duplicate-interval.md
new file mode 100644
index 000000000..771298721
--- /dev/null
+++ b/.changeset/fix-duplicate-interval.md
@@ -0,0 +1,5 @@
+---
+"@stackflow/core": patch
+---
+
+fix(core): prevent duplicate setInterval in dispatchEvent
diff --git a/core/src/makeCoreStore.ts b/core/src/makeCoreStore.ts
index a892b1d2b..0043055c5 100644
--- a/core/src/makeCoreStore.ts
+++ b/core/src/makeCoreStore.ts
@@ -84,7 +84,7 @@ export function makeCoreStore(options: MakeCoreStoreOptions): CoreStore {
     value: aggregate(events.value, new Date().getTime()),
   };
 
-  let intervalRunning = false;
+  let currentInterval: ReturnType<typeof setInterval> | null = null;
 
   const actions: StackflowActions = {
     getStack() {
@@ -100,12 +100,10 @@ export function makeCoreStore(options: MakeCoreStoreOptions): CoreStore {
       events.value.push(newEvent);
       setStackValue(nextStackValue);
 
-      if (intervalRunning) {
-        return;
+      if (currentInterval !== null) {
+        clearInterval(currentInterval);
       }
 
-      intervalRunning = true;
-
       const interval = setInterval(() => {
         const nextStackValue = aggregate(events.value, new Date().getTime());
 
@@ -115,9 +113,12 @@ export function makeCoreStore(options: MakeCoreStoreOptions): CoreStore {
 
         if (nextStackValue.globalTransitionState === "idle") {
           clearInterval(interval);
-          intervalRunning = false;
+          if (currentInterval === interval) {
+            currentInterval = null;
+          }
         }
       }, INTERVAL_MS);
+      currentInterval = interval;
     },
     push: () => {},
     replace: () => {},