diff --git a/packages/ui/src/shell/App.tsx b/packages/ui/src/shell/App.tsx
index ee813c1a2f..da3e9c62d8 100644
--- a/packages/ui/src/shell/App.tsx
+++ b/packages/ui/src/shell/App.tsx
@@ -26,6 +26,7 @@ import { BootstrapFallback } from "@posthog/ui/shell/BootstrapFallback";
 import { ErrorBoundary } from "@posthog/ui/shell/ErrorBoundary";
 import { openExternalUrl } from "@posthog/ui/shell/openExternal";
 import { useThemeStore } from "@posthog/ui/shell/themeStore";
+import { useAppVisibilityWatchdog } from "@posthog/ui/shell/useAppVisibilityWatchdog";
 import { Flex, Spinner, Text } from "@radix-ui/themes";
 import { RouterProvider } from "@tanstack/react-router";
 import { AnimatePresence, motion } from "framer-motion";
@@ -102,6 +103,17 @@ function App({ devToolbar }: AppProps) {
     setShowTransition(false);
   };
 
+  const mainRef = useRef<HTMLDivElement>(null);
+  // Mirrors the "main" branch of renderContent() below; keep the two in sync.
+  const showingMainApp =
+    isBootstrapped &&
+    isAuthenticated &&
+    hasCompletedOnboarding &&
+    !isCheckingAccess &&
+    !needsInviteCode &&
+    !needsAiApproval;
+  useAppVisibilityWatchdog(mainRef, showingMainApp);
+
   if (!isBootstrapped) {
     return <BootstrapFallback />;
   }
@@ -176,13 +188,7 @@ function App({ devToolbar }: AppProps) {
     }
 
     return (
-      <motion.div
-        key="main"
-        initial={{ opacity: 0 }}
-        animate={{ opacity: 1 }}
-        transition={{ duration: 0.5, delay: showTransition ? 0.5 : 0 }}
-        className="h-full"
-      >
+      <motion.div key="main" ref={mainRef} className="app-fade-in h-full">
         <RouterProvider router={router} />
         {/* Surfaces a toast when a backgrounded canvas generation finishes,
             from anywhere in the app. Sibling of the router so it stays mounted
diff --git a/packages/ui/src/shell/useAppVisibilityWatchdog.test.ts b/packages/ui/src/shell/useAppVisibilityWatchdog.test.ts
new file mode 100644
index 0000000000..14f76efa7f
--- /dev/null
+++ b/packages/ui/src/shell/useAppVisibilityWatchdog.test.ts
@@ -0,0 +1,135 @@
+import { renderHook } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@posthog/ui/shell/analytics", () => ({
+  captureException: vi.fn(),
+}));
+
+vi.mock("@posthog/ui/shell/logger", () => ({
+  logger: {
+    scope: () => ({
+      error: vi.fn(),
+      warn: vi.fn(),
+      info: vi.fn(),
+      debug: vi.fn(),
+    }),
+  },
+}));
+
+import { captureException } from "@posthog/ui/shell/analytics";
+import { useAppVisibilityWatchdog } from "./useAppVisibilityWatchdog";
+
+function mountElement(opacity: string, width: number, height: number) {
+  const element = document.createElement("div");
+  element.style.opacity = opacity;
+  element.getBoundingClientRect = () => ({ width, height }) as DOMRect;
+  document.body.append(element);
+  return element;
+}
+
+beforeEach(() => {
+  vi.useFakeTimers();
+});
+
+afterEach(() => {
+  vi.runOnlyPendingTimers();
+  vi.useRealTimers();
+  vi.mocked(captureException).mockClear();
+  document.body.innerHTML = "";
+});
+
+describe("useAppVisibilityWatchdog", () => {
+  it.each([
+    {
+      name: "invisible via opacity",
+      opacity: "0",
+      width: 1200,
+      height: 800,
+      active: true,
+      reports: true,
+    },
+    {
+      name: "collapsed to zero size",
+      opacity: "1",
+      width: 0,
+      height: 0,
+      active: true,
+      reports: true,
+    },
+    {
+      name: "visible",
+      opacity: "1",
+      width: 1200,
+      height: 800,
+      active: true,
+      reports: false,
+    },
+    {
+      name: "inactive",
+      opacity: "0",
+      width: 1200,
+      height: 800,
+      active: false,
+      reports: false,
+    },
+  ])(
+    "reports=$reports when $name",
+    ({ opacity, width, height, active, reports }) => {
+      const ref = { current: mountElement(opacity, width, height) };
+      renderHook(() => useAppVisibilityWatchdog(ref, active));
+
+      vi.advanceTimersByTime(3000);
+
+      expect(captureException).toHaveBeenCalledTimes(reports ? 1 : 0);
+    },
+  );
+
+  it("reports the element's opacity and source", () => {
+    const ref = { current: mountElement("0", 1200, 800) };
+    renderHook(() => useAppVisibilityWatchdog(ref, true));
+
+    vi.advanceTimersByTime(3000);
+
+    expect(captureException).toHaveBeenCalledWith(
+      expect.any(Error),
+      expect.objectContaining({
+        source: "app-visibility-watchdog",
+        opacity: 0,
+      }),
+    );
+  });
+
+  it("does not report after unmounting before the deadline", () => {
+    const ref = { current: mountElement("0", 1200, 800) };
+    const { unmount } = renderHook(() => useAppVisibilityWatchdog(ref, true));
+
+    unmount();
+    vi.advanceTimersByTime(3000);
+
+    expect(captureException).not.toHaveBeenCalled();
+  });
+
+  it("arms when active flips from false to true", () => {
+    const ref = { current: mountElement("0", 1200, 800) };
+    const { rerender } = renderHook(
+      ({ active }) => useAppVisibilityWatchdog(ref, active),
+      { initialProps: { active: false } },
+    );
+
+    vi.advanceTimersByTime(3000);
+    expect(captureException).not.toHaveBeenCalled();
+
+    rerender({ active: true });
+    vi.advanceTimersByTime(3000);
+    expect(captureException).toHaveBeenCalledOnce();
+  });
+
+  it("does nothing when the ref never attaches", () => {
+    const ref = { current: null };
+    renderHook(() => useAppVisibilityWatchdog(ref, true));
+
+    vi.advanceTimersByTime(3000);
+
+    expect(captureException).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/ui/src/shell/useAppVisibilityWatchdog.ts b/packages/ui/src/shell/useAppVisibilityWatchdog.ts
new file mode 100644
index 0000000000..af1253deac
--- /dev/null
+++ b/packages/ui/src/shell/useAppVisibilityWatchdog.ts
@@ -0,0 +1,36 @@
+import { captureException } from "@posthog/ui/shell/analytics";
+import { logger } from "@posthog/ui/shell/logger";
+import { type RefObject, useEffect } from "react";
+
+const log = logger.scope("app-visibility");
+const VISIBILITY_CHECK_DELAY_MS = 3000;
+
+// Detects the "white screen but app alive" state: mounted and interactive yet stuck invisible.
+export function useAppVisibilityWatchdog(
+  ref: RefObject<HTMLElement | null>,
+  active: boolean,
+): void {
+  useEffect(() => {
+    if (!active) return;
+    const timer = setTimeout(() => {
+      const element = ref.current;
+      if (!element) return;
+      const computedOpacity = getComputedStyle(element).opacity;
+      const opacity = computedOpacity ? Number.parseFloat(computedOpacity) : 1;
+      const rect = element.getBoundingClientRect();
+      if (opacity >= 0.01 && rect.width > 0 && rect.height > 0) return;
+      const detail = {
+        opacity,
+        width: Math.round(rect.width),
+        height: Math.round(rect.height),
+        route: window.location.hash,
+      };
+      log.error("Main app mounted but not visible", detail);
+      captureException(new Error("Main app mounted but not visible"), {
+        ...detail,
+        source: "app-visibility-watchdog",
+      });
+    }, VISIBILITY_CHECK_DELAY_MS);
+    return () => clearTimeout(timer);
+  }, [ref, active]);
+}
diff --git a/packages/ui/src/styles/globals.css b/packages/ui/src/styles/globals.css
index 4712bec6bd..d99735a7db 100644
--- a/packages/ui/src/styles/globals.css
+++ b/packages/ui/src/styles/globals.css
@@ -865,6 +865,25 @@ body {
   }
 }
 
+/* Resting opacity is 1, so a dropped/stalled animation can never leave the app invisible. */
+.app-fade-in {
+  animation: appFadeIn 0.5s ease-out forwards;
+}
+
+/* The window-blur freeze above pauses every keyframe animation; let this one-shot entrance finish anyway, or a blur mid-fade would strand the app invisible. */
+body.ph-window-blurred .app-fade-in {
+  animation-play-state: running !important;
+}
+
+@keyframes appFadeIn {
+  from {
+    opacity: 0;
+  }
+  to {
+    opacity: 1;
+  }
+}
+
 .tree-item-hover:hover {
   background-color: var(--gray-3);
 }