diff --git a/e2e/settings-overscroll.spec.ts b/e2e/settings-overscroll.spec.ts
index dc7ab9ec3..ecc4d8434 100644
--- a/e2e/settings-overscroll.spec.ts
+++ b/e2e/settings-overscroll.spec.ts
@@ -68,12 +68,22 @@ test.describe("settings + admin vertical over-scroll guard", () => {
         // long when the page is ready and too short on a loaded runner, and
         // it never says what it is waiting for. Gate on the element the
         // measurement reads instead.
-        await settleBeforeMeasure(page, page.locator("#main-content"));
+        const wrapperLocator = page.locator(
+          '#main-content [data-slot="main-content-wrapper"]',
+        );
+        await settleBeforeMeasure(page, wrapperLocator);
 
         const dims = await page.evaluate(() => {
           const main = document.getElementById("main-content");
           if (!main) return null;
-          const wrapper = main.firstElementChild as HTMLElement | null;
+          // Selected by the stable `data-slot` (auth-shell.tsx), not by
+          // birth order: `firstElementChild` would resolve to whatever
+          // lazily-mounted sibling (portal, toast, hydrating region) landed
+          // first, silently measuring the wrong subtree instead of the
+          // real content wrapper.
+          const wrapper = main.querySelector<HTMLElement>(
+            '[data-slot="main-content-wrapper"]',
+          );
           if (!wrapper) return null;
           // Lowest content edge: max CONTENT-box bottom over the wrapper's
           // visible, non-fixed descendants, in the scroll container's
diff --git a/e2e/v1341-navigation-and-insights-mobile.spec.ts b/e2e/v1341-navigation-and-insights-mobile.spec.ts
index 2c446c677..f01782f7d 100644
--- a/e2e/v1341-navigation-and-insights-mobile.spec.ts
+++ b/e2e/v1341-navigation-and-insights-mobile.spec.ts
@@ -21,25 +21,38 @@ test.describe("v1.34.1 navigation geometry", () => {
 
         const heading = page.getByRole("heading", { level: 1 });
         const sidebar = page.locator("aside").first();
+        // The wrapper carries a stable `data-slot` (auth-shell.tsx) so both
+        // reads below measure the same DOM node instead of resolving
+        // `firstElementChild` twice — a lazily-mounted sibling (portal,
+        // toast, hydrating region) landing ahead of it between the two
+        // reads used to substitute a different element into the second
+        // measurement, producing an unrelated number rather than a
+        // drifted one. Resolving the handle once removes the second
+        // resolution step entirely.
+        const wrapper = page.locator('[data-slot="main-content-wrapper"]');
         await settleBeforeMeasure(page, heading);
         await expect(sidebar).toBeVisible();
+        await expect(wrapper).toBeVisible();
+        const wrapperHandle = await wrapper.elementHandle();
+        if (!wrapperHandle) {
+          throw new Error("main content wrapper not found");
+        }
 
-        const before = await page.evaluate(() => {
+        const before = await page.evaluate((wrapperEl) => {
           const main = document.getElementById("main-content");
           const heading = main?.querySelector("h1");
           const aside = main?.querySelector("aside");
-          const wrapper = main?.firstElementChild;
-          if (!main || !heading || !aside || !wrapper) return null;
+          if (!main || !heading || !aside || !wrapperEl) return null;
           return {
             headingTop: heading.getBoundingClientRect().top,
             asideTop: aside.getBoundingClientRect().top,
-            wrapperLeft: wrapper.getBoundingClientRect().left,
+            wrapperLeft: wrapperEl.getBoundingClientRect().left,
             mainWidth: main.clientWidth,
             documentScrollable:
               document.documentElement.scrollHeight >
               document.documentElement.clientHeight + 1,
           };
-        });
+        }, wrapperHandle);
 
         expect(before).not.toBeNull();
         if (!before) return;
@@ -51,26 +64,33 @@ test.describe("v1.34.1 navigation geometry", () => {
         await page.locator("#main-content").evaluate((main) => {
           main.scrollTop = Math.min(320, main.scrollHeight - main.clientHeight);
         });
-        await page.waitForFunction(() => {
+        // Gate on the wrapper being present and laid out (not just on
+        // scroll state) — the measurement below reads it, so the wait
+        // must cover everything that measurement depends on.
+        await page.waitForFunction((wrapperEl) => {
           const main = document.getElementById("main-content");
+          if (!main) return true;
+          const wrapperReady =
+            !!wrapperEl &&
+            wrapperEl.isConnected &&
+            wrapperEl.getBoundingClientRect().width > 0;
           return (
-            !main ||
-            main.scrollTop > 0 ||
-            main.scrollHeight <= main.clientHeight
+            wrapperReady &&
+            (main.scrollTop > 0 || main.scrollHeight <= main.clientHeight)
           );
-        });
+        }, wrapperHandle);
 
-        const after = await page.evaluate(() => {
+        const after = await page.evaluate((wrapperEl) => {
           const main = document.getElementById("main-content");
           const aside = main?.querySelector("aside");
-          const wrapper = main?.firstElementChild;
-          if (!main || !aside || !wrapper) return null;
+          if (!main || !aside || !wrapperEl) return null;
           return {
             asideTop: aside.getBoundingClientRect().top,
-            wrapperLeft: wrapper.getBoundingClientRect().left,
+            wrapperLeft: wrapperEl.getBoundingClientRect().left,
             mainWidth: main.clientWidth,
           };
-        });
+        }, wrapperHandle);
+        await wrapperHandle.dispose();
 
         expect(after).not.toBeNull();
         if (!after) return;
diff --git a/src/components/layout/auth-shell.tsx b/src/components/layout/auth-shell.tsx
index ab3b440ee..5b1d8ecc9 100644
--- a/src/components/layout/auth-shell.tsx
+++ b/src/components/layout/auth-shell.tsx
@@ -335,7 +335,10 @@ export function AuthShell({
               the desktop bottom-6 anchor), so the last line of content
               can always scroll out from under the floating button.
             */}
-            <div className="mx-auto max-w-screen-xl px-4 pt-6 pb-20 md:px-6">
+            <div
+              data-slot="main-content-wrapper"
+              className="mx-auto max-w-screen-xl px-4 pt-6 pb-20 md:px-6"
+            >
               {children}
             </div>
           </main>