diff --git a/packages/app/e2e/browser/autocomplete-popover-anchor-staleness.spec.ts b/packages/app/e2e/browser/autocomplete-popover-anchor-staleness.spec.ts
new file mode 100644
index 0000000000..42b2d744c7
--- /dev/null
+++ b/packages/app/e2e/browser/autocomplete-popover-anchor-staleness.spec.ts
@@ -0,0 +1,140 @@
+import { expect, test, type Page } from "../support/fixtures";
+import { composerLocator, expectComposerVisible } from "../support/helpers/composer";
+import { openAgentRoute, seedMockAgentWorkspace } from "../support/helpers/mock-agent";
+import { daemonWsRoutePattern } from "../support/helpers/daemon-port";
+
+const TEST_COMMANDS = [
+  { name: "help", description: "Show help", argumentHint: "" },
+  { name: "tdd", description: "Red test first", argumentHint: "" },
+  { name: "hello", description: "Greeting", argumentHint: "" },
+] as const;
+
+async function installListCommandsStub(page: Page): Promise<void> {
+  await page.routeWebSocket(daemonWsRoutePattern(), (ws) => {
+    const server = ws.connectToServer();
+    ws.onMessage((message) => server.send(message));
+    server.onMessage((message) => {
+      if (typeof message !== "string") {
+        ws.send(message);
+        return;
+      }
+      try {
+        const parsed = JSON.parse(message) as {
+          type?: string;
+          message?: { type?: string; payload?: { commands?: unknown; error?: string | null } };
+        };
+        if (
+          parsed.type === "session" &&
+          parsed.message?.type === "list_commands_response" &&
+          parsed.message.payload
+        ) {
+          parsed.message.payload.commands = TEST_COMMANDS;
+          parsed.message.payload.error = null;
+          ws.send(JSON.stringify(parsed));
+          return;
+        }
+      } catch {
+        // forward
+      }
+      ws.send(message);
+    });
+  });
+}
+
+async function boxes(page: Page) {
+  const popover = await page.evaluate(() => {
+    const element = document.querySelector('[data-testid="composer-autocomplete-popover"]');
+    if (!(element instanceof HTMLElement)) return null;
+    const rect = element.getBoundingClientRect();
+    return {
+      x: Math.round(rect.x),
+      y: Math.round(rect.y),
+      width: Math.round(rect.width),
+      height: Math.round(rect.height),
+    };
+  });
+  const composerBox = await page.getByTestId("message-input-root").boundingBox();
+  return {
+    popover,
+    composer: composerBox
+      ? {
+          x: Math.round(composerBox.x),
+          y: Math.round(composerBox.y),
+          width: Math.round(composerBox.width),
+          height: Math.round(composerBox.height),
+        }
+      : null,
+  };
+}
+
+// Failing evidence for https://github.com/getpaseo/paseo/issues/4872: the slash
+// autocomplete popover measures its anchor once per slash session, so a layout change
+// under an open popover leaves it anchored to where the composer used to be.
+test.describe("autocomplete popover anchoring", () => {
+  test("follows the composer when the sidebar closes while the popover is open", async ({
+    page,
+  }) => {
+    await installListCommandsStub(page);
+    const session = await seedMockAgentWorkspace({
+      repoPrefix: "repro-4872-anchor-a-",
+      title: "Repro 4872 anchor A",
+    });
+
+    try {
+      await openAgentRoute(page, session);
+      await expectComposerVisible(page, { timeout: 30_000 });
+      const input = composerLocator(page);
+      await expect(input).toBeEditable({ timeout: 30_000 });
+      await input.click();
+      await page.keyboard.type("/");
+      await expect(page.getByText("/help", { exact: true })).toBeVisible({ timeout: 30_000 });
+
+      console.log("A BEFORE", JSON.stringify(await boxes(page)));
+      await page.getByTestId("menu-button").first().click();
+      await page.waitForTimeout(1200);
+      const after = await boxes(page);
+      console.log("A AFTER", JSON.stringify(after));
+
+      expect(
+        Math.abs((after.popover?.x ?? 0) - (after.composer?.x ?? 0)),
+        "popover stays anchored to the composer",
+      ).toBeLessThanOrEqual(4);
+    } finally {
+      await session.cleanup();
+    }
+  });
+
+  test("stays on screen when the sidebar opens while the popover is open", async ({ page }) => {
+    await installListCommandsStub(page);
+    const session = await seedMockAgentWorkspace({
+      repoPrefix: "repro-4872-anchor-b-",
+      title: "Repro 4872 anchor B",
+    });
+
+    try {
+      await openAgentRoute(page, session);
+      await expectComposerVisible(page, { timeout: 30_000 });
+
+      // Collapse the sidebar first, so the toggle under test expands it.
+      await page.getByTestId("menu-button").first().click();
+      await page.waitForTimeout(1200);
+
+      const input = composerLocator(page);
+      await expect(input).toBeEditable({ timeout: 30_000 });
+      await input.click();
+      await page.keyboard.type("/");
+      await expect(page.getByText("/help", { exact: true })).toBeVisible({ timeout: 30_000 });
+      console.log("B BEFORE", JSON.stringify(await boxes(page)));
+
+      await page.getByTestId("menu-button").first().click();
+      await page.waitForTimeout(1200);
+      const after = await boxes(page);
+      console.log("B AFTER", JSON.stringify(after), "viewport", JSON.stringify(page.viewportSize()));
+
+      const popover = page.getByTestId("composer-autocomplete-popover").first();
+      await expect(popover).toBeInViewport({ ratio: 0.9, timeout: 5_000 });
+    } finally {
+      await session.cleanup();
+    }
+  });
+});