Hide & Show the Fusion Shell

Lets a Flows app hide the Fusion sidebar + topbar (the "shell") to use the full browser viewport, and reveal it again — without leaving the user stranded.

Requires @cognite/app-sdk's connectToHostApp() handshake already wired up, and @cognite/app-sdk >= 0.9.0 (verify with npm ls @cognite/app-sdk or check package.json — earlier versions don't expose setHideShell on HostAppAPI at all). If auth isn't wired up yet, run the setup-flows-auth skill first — but make sure it lands on the Apps API flow (app.json has "infra": "appsApi", deployed with npx @cognite/cli@latest apps deploy), not the Classic flow (DuneAuthProvider/useDune() from @cognite/dune). setHideShell only exists on the Apps API's HostAppAPI; Classic apps have no equivalent and are deployed to infrastructure @cognite/cli itself refuses to touch ("Legacy infrastructure is no longer supported"). If app.json is missing infra: "appsApi" or @cognite/dune shows up in package.json, stop and migrate to Apps API first — don't attempt this skill on a Classic app.

What it does

HostAppAPI.setHideShell(hidden: boolean): Promise<void> (from the api object returned by connectToHostApp()):

• setHideShell(true) — hides the CDF sidebar and topbar, giving the app the full viewport.
• setHideShell(false) — reveals them again.

Under the hood this toggles a bookmarkable ?hideShell=true URL parameter — no server round-trip, and a shared link already opens in full-screen mode. The shell also auto-reveals if the user navigates away from your app, as a safety net — but don't rely on that as your only way back.

There is no other safety net. While the shell is hidden, Fusion does not render any floating "reveal" button of its own — the navrail (and any toggle button inside it) is unmounted along with the rest of the shell. The only ways back are: your app's own reveal control, manually editing the URL, or navigating away entirely. Treat the guidance below as non-negotiable, not a nice-to-have.

When to use it

Good fit:

• Your app renders its own side navigation, so the CDF sidebar is redundant screen real estate.
• The app needs the full canvas — a dashboard, drawing surface, kiosk-style view, etc.

Not a fit:

• Hiding the shell "by default" with no user action — always gate it behind an explicit, reversible interaction (a toggle the user clicks), never on mount.

Non-negotiable: always leave a way back

The #1 failure mode of this feature is trapping the user in full-screen with no visible way to get the CDF navigation back. Every setHideShell(true) call must ship with an equally discoverable reveal control:

• App has its own side nav — put a small "Show Cognite menu" control at the bottom of that nav, in the same spot the CDF sidebar's own collapse/expand toggle would be. This is the pattern the platform team converged on for Flows apps.
• App has no side nav — use a persistent, low-key icon button (e.g. fixed corner) that's always visible, not something that only appears on hover. Hover-only affordances don't work on touch/mobile.
• Icon-only toggles need an aria-label (e.g. "Hide Cognite menu" / "Show Cognite menu") — don't ship an icon button screen readers can't interpret. A button with visible label text already has an accessible name and doesn't need one.

Step 1 — Add the useHideShell hook

Create (or add to an existing hooks file) src/hooks/use-hide-shell.ts. This centralizes the toggle logic and — critically — restores the shell on unmount, so navigating within your own app (or an error boundary tearing down the tree) can never leave the shell permanently hidden:

import { useCallback, useEffect, useState } from 'react';
import type { HostAppAPI } from '@cognite/app-sdk';

/**
 * Manages Fusion shell visibility for full-screen "app-only" mode.
 *
 * Restores the shell automatically on unmount so it's never left hidden
 * if the user navigates away or the component tears down unexpectedly.
 */
export function useHideShell(api: HostAppAPI | null) {
  const [isHidden, setIsHidden] = useState(false);

  const setHidden = useCallback(
    async (next: boolean) => {
      if (!api) return;
      await api.setHideShell(next);
      setIsHidden(next);
    },
    [api],
  );

  const toggle = useCallback(() => setHidden(!isHidden), [setHidden, isHidden]);
  const hide = useCallback(() => setHidden(true), [setHidden]);
  const reveal = useCallback(() => setHidden(false), [setHidden]);

  useEffect(() => {
    return () => {
      if (isHidden && api) void api.setHideShell(false);
    };
  }, [api, isHidden]);

  return { isHidden, toggle, hide, reveal };
}

Step 2 — Add the toggle control

import type { HostAppAPI } from '@cognite/app-sdk';
import { Button } from '@cognite/aura/components';
import { IconEye, IconEyeOff } from '@tabler/icons-react';

import { useHideShell } from '../hooks/use-hide-shell';

function FullScreenToggle({ api }: { api: HostAppAPI | null }) {
  const { isHidden, toggle } = useHideShell(api);

  return (
    <Button variant="secondary" size="sm" onClick={toggle} disabled={!api}>
      {isHidden ? <IconEye aria-hidden /> : <IconEyeOff aria-hidden />}
      {isHidden ? 'Show Cognite menu' : 'Hide Cognite menu'}
    </Button>
  );
}

Place <FullScreenToggle api={api} /> wherever your best-practice placement (above) calls for it — bottom of your custom nav, or a fixed corner control.

IconEye/IconEyeOff read more clearly as a visibility toggle than arrows-style icons, which are easily confused with the separate native Fullscreen API. This matches the icon choice used in a working, deployed reference app for this skill (IconEyeOff to hide, IconEye to reveal).

Button comes from the @cognite/aura/components subpath — the package only exports that path (plus ./utils, ./eslint, ./styles.css), not a per-component @cognite/aura/components/button path. Importing the latter throws ERR_PACKAGE_PATH_NOT_EXPORTED at build time.

Step 3 — Add tests

Add tests alongside the hook at src/hooks/use-hide-shell.test.ts:

import { act, renderHook } from '@testing-library/react';
import { beforeEach, describe, expect, it, vi } from 'vitest';
import type { HostAppAPI } from '@cognite/app-sdk';

import { useHideShell } from './use-hide-shell';

function makeApi(): Pick<HostAppAPI, 'setHideShell'> {
  return { setHideShell: vi.fn(() => Promise.resolve()) };
}

describe('useHideShell', () => {
  let api: ReturnType<typeof makeApi>;

  beforeEach(() => {
    api = makeApi();
    vi.clearAllMocks();
  });

  it('starts with the shell visible', () => {
    const { result } = renderHook(() => useHideShell(api as HostAppAPI));
    expect(result.current.isHidden).toBe(false);
  });

  it('hides the shell on toggle', async () => {
    const { result } = renderHook(() => useHideShell(api as HostAppAPI));
    await act(() => result.current.toggle());
    expect(api.setHideShell).toHaveBeenCalledWith(true);
    expect(result.current.isHidden).toBe(true);
  });

  it('reveals the shell on the second toggle', async () => {
    const { result } = renderHook(() => useHideShell(api as HostAppAPI));
    await act(() => result.current.toggle());
    await act(() => result.current.toggle());
    expect(api.setHideShell).toHaveBeenLastCalledWith(false);
    expect(result.current.isHidden).toBe(false);
  });

  it('restores the shell on unmount when hidden', async () => {
    const { result, unmount } = renderHook(() => useHideShell(api as HostAppAPI));
    await act(() => result.current.hide());
    unmount();
    expect(api.setHideShell).toHaveBeenLastCalledWith(false);
  });

  it('does not call setHideShell on unmount when already visible', () => {
    const { unmount } = renderHook(() => useHideShell(api as HostAppAPI));
    unmount();
    expect(api.setHideShell).not.toHaveBeenCalled();
  });

  it('is a no-op when api is null (running outside Fusion)', async () => {
    const { result } = renderHook(() => useHideShell(null));
    await act(() => result.current.toggle());
    expect(result.current.isHidden).toBe(false);
  });
});

Step 4 — Handle the case where the shell doesn't hide

Three conditions must all be true for setHideShell(true) to actually hide anything:

1. The NAVIGATION_HIDE_SHELL Unleash flag is enabled for the environment.
2. Your app is the active route the shell recognizes as a managed Flows app (the Custom Apps subapp, with ?hideShell present in the URL).
3. Nothing else on the page has already re-shown the shell (e.g. the user navigated to a different Fusion area, which auto-reveals it).

Don't build extra fallback UI for the "flag off" case — the call is a no-op and the shell simply stays visible. Just don't assume the toggle always visibly does something in every environment while testing.

Step 5 — Test both directions

• Click "Hide Cognite menu" → shell disappears, app fills the viewport, the URL now has ?hideShell=true.
• Click "Show Cognite menu" → shell reappears, hideShell is removed from the URL.
• Reload the page with ?hideShell=true in the URL → shell starts hidden (bookmarkable).
• Navigate to an unrelated Fusion route while hidden → shell auto-reveals.

Matching the Fusion sidebar's width

If your own side nav needs to visually line up with (or replace) the CDF sidebar, match these widths from apps/navigation's navrail (apps/navigation/src/utils/constants.ts in cognitedata/fusion):

These are internal implementation details of the Fusion shell, not a public design-system token — re-check them if the sidebar's look changes noticeably, since there's no guarantee they stay in sync with this skill.

Related

• setup-flows-auth — prerequisite app-sdk connection.
• use-topbar — if your app renders its own topbar, the shell toggle above is unrelated to (and composes fine with) that in-app Aura Topbar.