diff --git a/.changeset/effect-mainline-ownership-3412.md b/.changeset/effect-mainline-ownership-3412.md
new file mode 100644
index 000000000..5a0784eb7
--- /dev/null
+++ b/.changeset/effect-mainline-ownership-3412.md
@@ -0,0 +1,5 @@
+---
+"@solidjs/signals": patch
+---
+
+Keep a mainline-computed effect value out of a parked transaction. When an effect stamped by a held transaction recomputed on an unrelated write and no longer read the held source, the forced re-run inside that transaction re-claimed ownership of the value it had just published. A finalize-time re-entry (a `Loading` boundary's `on` reset flipping its fallback state) then parked the effect with the transaction, leaving a `show() ? details() : "hidden"` reader stale until the unrelated async settled (#3412).
diff --git a/packages/signals/src/core/core.ts b/packages/signals/src/core/core.ts
index 0567cc92c..a50c5f46a 100644
--- a/packages/signals/src/core/core.ts
+++ b/packages/signals/src/core/core.ts
@@ -653,10 +653,15 @@ export function recompute(el: Computed<any>, create: boolean = false): void {
     (!create || el._statusFlags & STATUS_PENDING) &&
     (!el._transition || hasOverride) &&
     queuePendingNode(el);
-  el._transition &&
-    isEffect &&
-    activeTransition !== el._transition &&
+  if (el._transition && isEffect && activeTransition !== el._transition) {
+    // The re-run refreshes the transaction's STAGED view (_pendingValue); the
+    // value this pass published in _value belongs to the run that just
+    // finished. Keep that ownership, or the effect phase parks a
+    // mainline-computed value with the transaction (#3412).
+    const owner = (el as any)._valueTransition;
     runInTransition(el._transition, () => recompute(el));
+    (el as any)._valueTransition = owner;
+  }
   // Missed-wake reschedule (see the finally above): values this pass read
   // before the nested commit are stale, so run again now that the heap will
   // accept the node. Equality gates stop same-value landings from cascading,
diff --git a/packages/signals/tests/effect-mainline-ownership-3412.test.ts b/packages/signals/tests/effect-mainline-ownership-3412.test.ts
new file mode 100644
index 000000000..7a7d7c5f6
--- /dev/null
+++ b/packages/signals/tests/effect-mainline-ownership-3412.test.ts
@@ -0,0 +1,110 @@
+/**
+ * #3412: an effect stamped by a parked transaction (it was pending on a held
+ * async source) recomputes mainline on an unrelated write and no longer reads
+ * the held source. The forced re-run inside its own transaction refreshes the
+ * staged view but must not claim ownership of the mainline value; if a
+ * finalize-time re-entry (a Loading `on` reset's `_disabled` write) makes the
+ * transaction active before the effect phase, the mainline value is otherwise
+ * parked with it.
+ */
+import { describe, expect, test, beforeEach, afterEach, vi } from "vitest";
+import {
+  createLoadingBoundary,
+  createMemo,
+  createRenderEffect,
+  createRoot,
+  createSignal,
+  flush
+} from "../src/index.js";
+
+beforeEach(() => vi.useFakeTimers());
+afterEach(() => vi.useRealTimers());
+
+const delay = <T>(ms: number, value: T) => new Promise<T>(r => setTimeout(r, ms, value));
+
+function setup(opts: { boundary: boolean; on: boolean; unconditional: boolean }) {
+  const out: Record<string, unknown> = {};
+  let setCount!: (v: number) => void;
+  let setShow!: (v: boolean) => void;
+  createRoot(() => {
+    const [count, _setCount] = createSignal(0);
+    const [show, _setShow] = createSignal(true);
+    setCount = _setCount;
+    setShow = _setShow;
+    const copy = createMemo(async () => count(), undefined, { name: "copy" });
+    const details = createMemo(() => delay(1500, copy()), undefined, { name: "details" });
+    createRenderEffect(
+      () => String(show()),
+      v => {
+        out.show = v;
+      },
+      { name: "E:show" }
+    );
+    if (opts.unconditional)
+      createRenderEffect(
+        () => details(),
+        v => {
+          out.details = v;
+        },
+        { name: "E:details" }
+      );
+    createRenderEffect(
+      () => (show() ? details() : "hidden"),
+      v => {
+        out.panel = v;
+      },
+      { name: "E:panel" }
+    );
+    if (opts.boundary) {
+      const b = createLoadingBoundary(
+        () => copy(),
+        () => "Loading...",
+        opts.on ? { on: () => count() } : undefined
+      );
+      createRenderEffect(
+        () => b(),
+        v => {
+          out.copy = v;
+        },
+        { name: "E:copy" }
+      );
+    } else {
+      createRenderEffect(
+        () => copy(),
+        v => {
+          out.copy = v;
+        },
+        { name: "E:copy" }
+      );
+    }
+  });
+  return { out, setCount, setShow };
+}
+
+describe("effect mainline ownership (#3412)", () => {
+  for (const opts of [
+    { boundary: true, on: true, unconditional: true },
+    { boundary: true, on: false, unconditional: true },
+    { boundary: false, on: false, unconditional: true },
+    { boundary: true, on: true, unconditional: false }
+  ]) {
+    test(`panel hides when show flips ${JSON.stringify(opts)}`, async () => {
+      const { out, setCount, setShow } = setup(opts);
+      flush();
+      await vi.advanceTimersByTimeAsync(1500);
+      flush();
+      setCount(1);
+      flush();
+      await vi.advanceTimersByTimeAsync(500);
+      flush();
+      setShow(false);
+      flush();
+      await Promise.resolve();
+      flush();
+      const afterShow = { ...out };
+      await vi.advanceTimersByTimeAsync(1500);
+      flush();
+      expect(afterShow.panel).toBe("hidden");
+    });
+  }
+});
diff --git a/packages/web/test/loading-on-outside-reader-3412.spec.tsx b/packages/web/test/loading-on-outside-reader-3412.spec.tsx
new file mode 100644
index 000000000..097003ed2
--- /dev/null
+++ b/packages/web/test/loading-on-outside-reader-3412.spec.tsx
@@ -0,0 +1,77 @@
+/**
+ * @jsxImportSource @solidjs/web
+ * @vitest-environment jsdom
+ */
+import { describe, expect, test, beforeEach, afterEach, vi } from "vitest";
+import { createSignal, createMemo, Loading, flush } from "solid-js";
+import { render } from "../src/index.js";
+
+beforeEach(() => vi.useFakeTimers());
+afterEach(() => vi.useRealTimers());
+
+const delay = <T = void,>(ms: number, value?: T) => new Promise<T>(r => setTimeout(r, ms, value));
+
+function setup(opts: { boundary: boolean; on: boolean; unconditional: boolean }) {
+  const div = document.createElement("div");
+  let setCount!: (v: number) => void;
+  let setShow!: (v: boolean) => void;
+  const dispose = render(() => {
+    const [count, _setCount] = createSignal(0);
+    const [show, _setShow] = createSignal(true);
+    setCount = _setCount;
+    setShow = _setShow;
+    const copy = createMemo(async () => count());
+    const details = createMemo(() => delay(1500, copy()));
+    return (
+      <>
+        <p>Show: {String(show())}</p>
+        {opts.unconditional ? <p>Details: {details()}</p> : null}
+        <p>Panel: {show() ? details() : "hidden"}</p>
+        <p>
+          Copy:{" "}
+          {opts.boundary ? (
+            opts.on ? (
+              <Loading on={count()} fallback="Loading...">
+                {copy()}
+              </Loading>
+            ) : (
+              <Loading fallback="Loading...">{copy()}</Loading>
+            )
+          ) : (
+            copy()
+          )}
+        </p>
+      </>
+    );
+  }, div);
+  return { div, setCount, setShow, dispose };
+}
+
+describe("Loading on reset with an outside conditional reader (#3412)", () => {
+  for (const opts of [
+    { boundary: true, on: true, unconditional: true },
+    { boundary: true, on: false, unconditional: true },
+    { boundary: false, on: false, unconditional: true },
+    { boundary: true, on: true, unconditional: false }
+  ]) {
+    test(`panel hides when show flips ${JSON.stringify(opts)}`, async () => {
+      const { div, setCount, setShow, dispose } = setup(opts);
+      flush();
+      await vi.advanceTimersByTimeAsync(1500);
+      flush();
+      setCount(1);
+      flush();
+      await vi.advanceTimersByTimeAsync(500);
+      flush();
+      setShow(false);
+      flush();
+      await Promise.resolve();
+      flush();
+      const afterShow = div.textContent;
+      await vi.advanceTimersByTimeAsync(1500);
+      flush();
+      expect(afterShow).toContain("Panel: hidden");
+      dispose();
+    });
+  }
+});
diff --git a/scripts/size/.size-limit.js b/scripts/size/.size-limit.js
index 3fb9ef302..50afb9b49 100644
--- a/scripts/size/.size-limit.js
+++ b/scripts/size/.size-limit.js
@@ -859,7 +859,11 @@ module.exports = [
     // 28268 (+32 — the core seams and the collecting boundary).
     // Boundary reset ends the hold (#3375 ruling, 2026-09-12): 28.35 -> 28.40 KB,
     // measured at 28380 B against 28300 (+80); see the core floor note.
-    limit: "28.40 KB",
+    // Mainline effect ownership across the forced in-transaction re-run
+    // (#3412, 2026-09-13): 28.40 -> 28.45 KB, measured at 28408 B on the
+    // merge with `next` — the save/restore of `_valueTransition` in
+    // recompute, on top of #3413's companion-gate change.
+    limit: "28.45 KB",
     modifyEsbuildConfig
   },
   {