fix(browser): preserve backslashes in POSIX file paths (#42257)

On macOS and Linux a backslash is an ordinary filename character, but
absolutePathToFileUrl converted every backslash into a path separator, so
typing /tmp/site\draft.html opened /tmp/site/draft.html instead.

Keep backslashes (encoded as %5C) for rooted paths when the local host is not
Windows. On Windows every path still has backslashes normalized to slashes,
so drive, UNC and rooted /foo\bar inputs behave as before.

Fixes #42256

Co-authored-by: Neil <neil@stably.ai>

src/shared/browser-url.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 
 import {
   isEligibleLocalCertificateHost,
@@ -43,6 +43,30 @@ describe('browser-url helpers', () => {
     expect(normalizeExternalBrowserUrl('about:blank')).toBeNull()
   })
 
+  describe('file path backslashes', () => {
+    afterEach(() => {
+      vi.unstubAllGlobals()
+    })
+
+    it('preserves backslashes as filename characters on POSIX hosts', () => {
+      vi.stubGlobal('navigator', { userAgent: 'Linux' })
+      expect(normalizeBrowserNavigationUrl('/tmp/site\\draft.html')).toBe(
+        'file:///tmp/site%5Cdraft.html'
+      )
+    })
+
+    it('normalizes backslashes to separators on Windows hosts', () => {
+      vi.stubGlobal('navigator', { userAgent: 'Windows' })
+      expect(normalizeBrowserNavigationUrl('/foo\\bar.html')).toBe('file:///foo/bar.html')
+      expect(normalizeBrowserNavigationUrl('C:\\site\\draft.html')).toBe(
+        'file:///C:/site/draft.html'
+      )
+      expect(normalizeBrowserNavigationUrl('\\\\server\\share\\draft.html')).toBe(
+        'file://server/share/draft.html'
+      )
+    })
+  })
+
   // Refuse file URLs externally: a remote page must not drive Finder/Explorer to arbitrary paths.
   it('rejects file:// for external opens even though it is allowed in-app', () => {
     expect(normalizeExternalBrowserUrl('file:///etc/passwd')).toBeNull()

src/shared/browser-url.ts

@@ -1,5 +1,6 @@
 import './constants'
 import { ORCA_BROWSER_BLANK_URL } from './browser/browser-blank-page'
+import { getCurrentLocalPlatform } from './execution-host'
 
 const LOCAL_ADDRESS_PATTERN =
   /^(?:localhost|127(?:\.\d{1,3}){3}|0\.0\.0\.0|\[[0-9a-f:]+\])(?::\d+)?(?:[/?#].*)?$/i
@@ -245,7 +246,9 @@ export function isAbsoluteFilesystemPathInput(input: string): boolean {
 }
 
 function absolutePathToFileUrl(filePath: string): string {
-  const normalizedPath = filePath.replaceAll('\\', '/')
+  // Why: `\` is a filename char on POSIX but a separator on Windows, even in rooted paths like `/foo\bar`.
+  const isPosixPath = filePath.startsWith('/') && getCurrentLocalPlatform() !== 'win32'
+  const normalizedPath = isPosixPath ? filePath : filePath.replaceAll('\\', '/')
   const segments = normalizedPath.split('/').map((segment, index) => {
     if (index === 0 && /^[A-Za-z]:$/.test(segment)) {
       return segment

src/shared/execution-host.ts

@@ -23,7 +23,7 @@ export type ParsedExecutionHost =
   | { kind: 'ssh'; id: `ssh:${string}`; targetId: string }
   | { kind: 'runtime'; id: `runtime:${string}`; environmentId: string }
 
-function getCurrentLocalPlatform(): NodeJS.Platform | null {
+export function getCurrentLocalPlatform(): NodeJS.Platform | null {
   const globalNavigator = (globalThis as { navigator?: { userAgent?: string; platform?: string } })
     .navigator
   const userAgent = globalNavigator?.userAgent || globalNavigator?.platform || ''