playwright: add a few platform-specific tests

The Git home page displays some platform-dependent behavior. With this
commit, we test that by pretending to run on the three major operating
systems: Windows, macOS and Linux. Specifically, we verify that:

- the correct download is suggested, based on the current browser

- a subset of the GUIs are shown based on the platform indicated by the
  current browser

Pretending to be running on a given platform is a bit tricky because the
`navigator.userAgent`/`navigator.platform` attributes are used by the
Git home page (via `session.min.js`) to determine what operating system
the user is running.

These can be easily overridden on most browser, using the
`Object.defineProperty()` method.

However, in my tests, Chromium on Linux (and only on that operating
system) simply ignored those calls and still reported that it is running
on Linux.

To work around this, we need to use the Chrome DevTools Protocol, via
the `BrowserContext.newCDPSession()` method; For more details see
https://playwright.dev/docs/api/class-browsercontext#browser-context-new-cdp-session

Obviously, this can _only_ be used with Chromium/Chrome and won't work
on, say, Firefox or Safari. That is the reason for the somewhat ugly
`pretendPlatform()` function we need to use here.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

tests/git-scm.spec.js

@@ -2,6 +2,80 @@ const { test, expect, selectors } = require('@playwright/test')
 
 const url = 'https://git.github.io/git-scm.com/'
 
+async function pretendPlatform(page, browserName, userAgent, platform) {
+  if (browserName !== 'chromium') {
+    await page.context().addInitScript({
+      content: `Object.defineProperty(navigator, 'platform', { get: () => '${platform}' })`
+    })
+  } else {
+    // As of time of writing, Chromium on Linux (but not on Windows or macOS)
+    // refuses to let the `navigator.platform` attribute to be overridden via
+    // `Object.defineProperty()`, therefore we use the Chrome DevTools Protocol
+    // with that browser (and only with that browser because it is proprietary
+    // to that browser).
+    const cdpSession = await page.context().newCDPSession(page);
+    await cdpSession.send('Emulation.setUserAgentOverride', { platform, userAgent });
+  }
+}
+
+test.describe('Windows', () => {
+  const userAgent = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36'
+  test.use({ userAgent })
+
+  test('download/GUI links', async ({ page, browserName }) => {
+    await pretendPlatform(page, browserName, userAgent, 'Windows')
+    await page.goto(url)
+    await expect(page.getByRole('link', { name: 'Download for Windows' })).toBeVisible()
+
+    await expect(page.getByRole('link', { name: 'Graphical UIs' })).toBeHidden()
+    const windowsGUIs = page.getByRole('link', { name: 'Windows GUIs' })
+    await expect(windowsGUIs).toBeVisible()
+    await expect(windowsGUIs).toHaveAttribute('href', /\/download\/gui\/windows$/)
+
+    // navigate to Windows GUIs
+    await windowsGUIs.click()
+    const windowsButton = page.getByRole('link', { name: 'Windows' })
+    await expect(windowsButton).toBeVisible()
+    await expect(windowsButton).toHaveClass(/selected/)
+
+    const allButton = page.getByRole('link', { name: 'All' })
+    await expect(allButton).not.toHaveClass(/selected/)
+
+    const thumbnails = page.locator('.gui-thumbnails li:visible')
+    const count = await thumbnails.count()
+    await allButton.click()
+    await expect.poll(() => thumbnails.count()).toBeGreaterThan(count)
+  })
+})
+
+test.describe('macOS', () => {
+  const userAgent = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 14_6_1) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.6 Safari/605.1.15'
+  test.use({ userAgent })
+
+  test('download/GUI links', async ({ page, browserName }) => {
+    await pretendPlatform(page, browserName, userAgent, 'Mac OS X')
+    await page.goto(url)
+    await expect(page.getByRole('link', { name: 'Download for Mac' })).toBeVisible()
+
+    await expect(page.getByRole('link', { name: 'Graphical UIs' })).toBeHidden()
+    await expect(page.getByRole('link', { name: 'Mac GUIs' })).toBeVisible()
+  })
+})
+
+test.describe('Linux', () => {
+  const userAgent = 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:130.0) Gecko/20100101 Firefox/130.0'
+  test.use({ userAgent })
+
+  test('download/GUI links', async ({ page, browserName }) => {
+    await pretendPlatform(page, browserName, userAgent, 'Linux')
+    await page.goto(url)
+    await expect(page.getByRole('link', { name: 'Download for Linux' })).toBeVisible()
+
+    await expect(page.getByRole('link', { name: 'Graphical UIs' })).toBeHidden()
+    await expect(page.getByRole('link', { name: 'Linux GUIs' })).toBeVisible()
+  })
+})
+
 test('search', async ({ page }) => {
   await page.goto(url)