test(core): add unit tests for RedirectUnsupportedBrowsers

Covers the main logic paths: supported browser (no redirect), unsupported
browser (redirect with btoa-encoded URL), already on the warning page
(no double redirect), and a no-throw guard that would have caught the
window.Buffer regression.

AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

core/src/tests/utils/RedirectUnsupportedBrowsers.spec.ts

@@ -0,0 +1,114 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { testSupportedBrowser } from '../../utils/RedirectUnsupportedBrowsers.js'
+
+// Mock the router so generateUrl returns a predictable path
+vi.mock('@nextcloud/router', () => ({
+	generateUrl: (path: string) => `/index.php${path}`,
+}))
+
+// Mock the logger to suppress output
+vi.mock('../../logger.js', () => ({
+	default: { debug: vi.fn() },
+}))
+
+const browserStorage = vi.hoisted(() => ({ getItem: vi.fn(() => null) }))
+vi.mock('../../services/BrowserStorageService.js', () => ({ default: browserStorage }))
+
+const supportedUA = 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/134.0.0.0 Safari/537.36'
+const unsupportedUA = 'Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)'
+
+describe('testSupportedBrowser', () => {
+	let originalLocation: Location
+
+	beforeEach(() => {
+		originalLocation = window.location
+
+		// Reset the override flag
+		browserStorage.getItem.mockReturnValue(null)
+
+		// Default to a path that isn't the unsupported-browser page
+		Object.defineProperty(window, 'location', {
+			configurable: true,
+			writable: true,
+			value: {
+				href: 'http://localhost/apps/files',
+				origin: 'http://localhost',
+				pathname: '/apps/files',
+				reload: vi.fn(),
+			},
+		})
+
+		vi.spyOn(window.history, 'pushState').mockImplementation(() => {})
+	})
+
+	afterEach(() => {
+		Object.defineProperty(window, 'location', {
+			configurable: true,
+			writable: true,
+			value: originalLocation,
+		})
+		vi.restoreAllMocks()
+	})
+
+	it('does nothing for a supported browser', () => {
+		Object.defineProperty(window.navigator, 'userAgent', { configurable: true, value: supportedUA })
+
+		testSupportedBrowser()
+
+		expect(window.history.pushState).not.toHaveBeenCalled()
+		expect(window.location.reload).not.toHaveBeenCalled()
+	})
+
+	it('redirects an unsupported browser to the warning page', () => {
+		Object.defineProperty(window.navigator, 'userAgent', { configurable: true, value: unsupportedUA })
+
+		testSupportedBrowser()
+
+		expect(window.history.pushState).toHaveBeenCalledOnce()
+		const [, , url] = (window.history.pushState as ReturnType<typeof vi.fn>).mock.calls[0]
+		expect(url).toMatch(/^\/index\.php\/unsupported\?redirect_url=/)
+		expect(window.location.reload).toHaveBeenCalledOnce()
+	})
+
+	it('encodes the redirect URL with btoa, not window.Buffer', () => {
+		Object.defineProperty(window.navigator, 'userAgent', { configurable: true, value: unsupportedUA })
+
+		testSupportedBrowser()
+
+		const [, , url] = (window.history.pushState as ReturnType<typeof vi.fn>).mock.calls[0]
+		const encoded = new URL(`http://localhost${url}`).searchParams.get('redirect_url')
+		expect(encoded).toBe(btoa('/apps/files'))
+	})
+
+	it('does not throw regardless of override flag state', () => {
+		// isBrowserOverridden is read at module-load time so the mock won't flip it
+		// retroactively — but we can at least assert the function never throws,
+		// which is the regression guard for the window.Buffer removal.
+		Object.defineProperty(window.navigator, 'userAgent', { configurable: true, value: unsupportedUA })
+		expect(() => testSupportedBrowser()).not.toThrow()
+	})
+
+	it('does not redirect when already on the unsupported-browser page', () => {
+		Object.defineProperty(window.navigator, 'userAgent', { configurable: true, value: unsupportedUA })
+		Object.defineProperty(window, 'location', {
+			configurable: true,
+			writable: true,
+			value: {
+				href: 'http://localhost/index.php/unsupported',
+				origin: 'http://localhost',
+				pathname: '/index.php/unsupported',
+				reload: vi.fn(),
+			},
+		})
+
+		testSupportedBrowser()
+
+		expect(window.history.pushState).not.toHaveBeenCalled()
+		expect(window.location.reload).not.toHaveBeenCalled()
+	})
+})