diff --git a/e2e/tests/journeys.spec.ts b/e2e/tests/journeys.spec.ts
index f290eae..4ac1f72 100644
--- a/e2e/tests/journeys.spec.ts
+++ b/e2e/tests/journeys.spec.ts
@@ -478,3 +478,36 @@ test('rankings stay populated after picking a specific year', async ({ page }) =
   await expect(rows.first()).toBeVisible({ timeout: 15_000 });
   expect(await rows.count()).toBeGreaterThan(5);
 });
+
+test('compare metric-help popover stays within the mobile viewport', async ({ page }) => {
+  await page.setViewportSize({ width: 390, height: 844 });
+  const [urn0, urn1] = await twoPrimaryUrns(page);
+  await page.goto(`/compare?urns=${urn0},${urn1}`);
+
+  await expect(
+    page.getByRole('heading', { name: 'At a glance' }),
+  ).toBeVisible({ timeout: 15_000 });
+
+  // The metric-help triggers are the circled-"?" buttons in the row labels.
+  // "More information" is InfoPopover's default accessible name.
+  const help = page.getByRole('button', { name: 'More information' }).first();
+  await expect(help).toBeVisible({ timeout: 15_000 });
+  await help.click();
+
+  const tip = page.getByRole('tooltip');
+  await expect(tip).toBeVisible();
+
+  // The whole bubble must sit inside the viewport — the original bug pushed it
+  // off the right edge with no way to scroll to it.
+  const box = await tip.boundingBox();
+  const width = page.viewportSize()!.width;
+  expect(box).not.toBeNull();
+  expect(box!.x).toBeGreaterThanOrEqual(0);
+  expect(box!.x + box!.width).toBeLessThanOrEqual(width);
+
+  // And the page must not have gained a horizontal scrollbar from the bubble.
+  const bodyOverflowsX = await page
+    .locator('body')
+    .evaluate((el) => el.scrollWidth > el.clientWidth + 1);
+  expect(bodyOverflowsX).toBe(false);
+});