Set committed value for computations created during transition

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;

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();
+  });
+});