From 22bd4969ba98f65ce53cb4b8eb53f20b187c7078 Mon Sep 17 00:00:00 2001
From: "Jacques P. du Toit" <jpdutoit@gmail.com>
Date: Wed, 11 Mar 2026 21:50:00 +0100
Subject: [PATCH 1/2] Set committed value for computations created during
 transition

Fixes #2046
---
 packages/solid/src/reactive/signal.ts       |  2 +
 packages/solid/web/test/transition.spec.tsx | 72 +++++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 packages/solid/web/test/transition.spec.tsx

diff --git a/packages/solid/src/reactive/signal.ts b/packages/solid/src/reactive/signal.ts
index ddce76160..97caeaa4c 100644
--- a/packages/solid/src/reactive/signal.ts
+++ b/packages/solid/src/reactive/signal.ts
@@ -1417,6 +1417,8 @@ function runComputation(node: Computation<any>, value: any, time: number) {
     if (node.updatedAt != null && "observers" in node) {
       writeSignal(node as Memo<any>, nextValue, true);
     } else if (Transition && Transition.running && node.pure) {
+      // On first computation during transition, also set committed value #2046
+      if (!Transition.sources.has(node as Memo<any>)) node.value = nextValue;
       Transition.sources.add(node as Memo<any>);
       (node as Memo<any>).tValue = nextValue;
     } else node.value = nextValue;
diff --git a/packages/solid/web/test/transition.spec.tsx b/packages/solid/web/test/transition.spec.tsx
new file mode 100644
index 000000000..064aa4eaa
--- /dev/null
+++ b/packages/solid/web/test/transition.spec.tsx
@@ -0,0 +1,72 @@
+/**
+ * @jsxImportSource solid-js
+ * @vitest-environment jsdom
+ */
+
+import { describe, expect, test } from "vitest";
+import { createSignal, createMemo, createResource, useTransition } from "../../src/index.js";
+import { render, Suspense } from "../src/index.js";
+
+describe("Transition memo stale read (#2046)", () => {
+  test("memo created during transition should not return undefined in committed state", async () => {
+    const div = document.createElement("div");
+    const [route, setRoute] = createSignal("home");
+    const [dbVersion, setDbVersion] = createSignal(1);
+    const [pending, start] = useTransition();
+    let dataRef: (() => { q: number }) | null = null;
+    let resolveResource: (v: string) => void;
+
+    function RouteComponent() {
+      // Always returns {q:42}. Never undefined.
+      const data = createMemo(() => ({ q: 42 }));
+      // Reads both dbVersion (external signal) and data
+      const label = createMemo(() => dbVersion() + ": " + data()!.q);
+      dataRef = data;
+      return <p>{label()}</p>;
+    }
+
+    let fetchCount = 0;
+    const dispose = render(() => {
+      const [resource] = createResource(
+        () => route(),
+        r => {
+          fetchCount++;
+          // First fetch resolves immediately
+          if (fetchCount <= 1) return Promise.resolve(r);
+          // Second fetch (during transition) stays pending
+          return new Promise<string>(resolve => {
+            resolveResource = resolve;
+          });
+        }
+      );
+      return (
+        <Suspense fallback="loading">
+          <p>{resource()}</p>
+          {route() === "detail" && <RouteComponent />}
+        </Suspense>
+      );
+    }, div);
+
+    // Wait for initial resource to resolve
+    await Promise.resolve();
+    await Promise.resolve();
+
+    // Navigate via transition — resource refetches, keeps transition pending
+    start(() => setRoute("detail"));
+    await Promise.resolve();
+    await Promise.resolve();
+    await Promise.resolve();
+
+    // RouteComponent mounted during transition, transition is pending
+    expect(dataRef).not.toBeNull();
+    expect(pending()).toBe(true);
+
+    // External signal change while transition is pending.
+    // label recomputes → reads data() → should be {q:42}, not undefined.
+    setDbVersion(2);
+    expect(dataRef!()).toEqual({ q: 42 });
+
+    resolveResource!("done");
+    dispose();
+  });
+});

From a33416386db0a3bf34c7b06b69dafa7bb6458b57 Mon Sep 17 00:00:00 2001
From: Ryan Carniato <ryansolid@gmail.com>
Date: Tue, 24 Mar 2026 11:58:58 -0700
Subject: [PATCH 2/2] Add changeset for solid-js patch

Set committed value for computations created during transition
---
 .changeset/heavy-grapes-sin.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/heavy-grapes-sin.md

diff --git a/.changeset/heavy-grapes-sin.md b/.changeset/heavy-grapes-sin.md
new file mode 100644
index 000000000..516222c31
--- /dev/null
+++ b/.changeset/heavy-grapes-sin.md
@@ -0,0 +1,5 @@
+---
+"solid-js": patch
+---
+
+Set committed value for computations created during transition