fix(desktop): harden workspace watcher fallback (#352)

## Summary

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: mussonking

apps/desktop/src/main/workspace-watcher.test.ts

@@ -124,10 +124,10 @@ describe('files-watcher subscribe / unsubscribe', () => {
     expect(watchMock).not.toHaveBeenCalled();
   });
 
-  it('rejects when the watcher cannot start', () => {
+  it('rejects when the watcher throws a non-system error', () => {
     reset();
     watchMock.mockImplementation(() => {
-      throw new Error('watch denied');
+      throw new Error('watcher implementation bug');
     });
     getDesignMock.mockReturnValue({
       id: 'd1',
@@ -136,21 +136,24 @@ describe('files-watcher subscribe / unsubscribe', () => {
     registerFilesWatcherIpc({} as never, () => null);
     const sub = getHandler('codesign:files:v1:subscribe');
 
-    const err = captureError(() => sub(null, { schemaVersion: 1, designId: 'd1' }));
+    const caught = captureError(() => sub(null, { schemaVersion: 1, designId: 'd1' }));
 
-    expect(err).toMatchObject({ name: 'CodesignError', code: 'IPC_DB_ERROR' });
+    expect(caught).toMatchObject({ name: 'CodesignError', code: 'IPC_DB_ERROR' });
     expect(watchMock).toHaveBeenCalledTimes(1);
   });
 
-  it('does not reject when the bound workspace folder is missing', () => {
+  it.each([
+    'ENOENT',
+    'ENOTDIR',
+  ])('does not reject when the bound workspace is unavailable with %s', (code) => {
     reset();
-    const err = Object.assign(new Error('no such file or directory'), { code: 'ENOENT' });
+    const err = Object.assign(new Error('workspace unavailable'), { code });
     watchMock.mockImplementation(() => {
       throw err;
     });
     getDesignMock.mockReturnValue({
       id: 'd1',
-      workspacePath: tempWorkspace('codesign-watch-missing'),
+      workspacePath: tempWorkspace(`codesign-watch-${code.toLowerCase()}`),
     });
     registerFilesWatcherIpc({} as never, () => null);
     const sub = getHandler('codesign:files:v1:subscribe');
@@ -162,11 +165,15 @@ describe('files-watcher subscribe / unsubscribe', () => {
   });
 
   it.each([
-    ['permission denial', 'EPERM'],
-    ['unsupported directory watch', 'EISDIR'],
-  ])('falls back to polling when native watch fails from %s', (_reason, code) => {
+    'EPERM',
+    'EACCES',
+    'EISDIR',
+    'EINVAL',
+    'ENOSPC',
+    'ERR_FEATURE_UNAVAILABLE_ON_PLATFORM',
+  ])('falls back to polling when native watch fails with %s', (code) => {
     reset();
-    const err = Object.assign(new Error('watch unavailable'), { code });
+    const err = Object.assign(new Error('native watch unavailable'), { code });
     watchMock.mockImplementation(() => {
       throw err;
     });

apps/desktop/src/main/workspace-watcher.ts

@@ -25,7 +25,7 @@ import { isIgnoredWorkspacePath } from './workspace-reader';
  * are coalesced into a single emit per 250ms so a `pnpm install` in the
  * workspace doesn't spam IPC.
  *
- * Uses `node:fs.watch({recursive: true})` — works on macOS (FSEvents) and
+ * Uses `node:fs.watch({recursive: true})` -- works on macOS (FSEvents) and
  * Linux (recent kernel). No chokidar dep; Windows recursive coverage is
  * weaker but we're macOS-first.
  */
@@ -36,7 +36,7 @@ const log = getLogger('files-watcher');
 const COALESCE_MS = 250;
 /** Keep an idle watcher alive briefly so quick tab-switches don't churn. */
 const IDLE_TEARDOWN_MS = 5 * 60_000;
-/** Permission-constrained Windows folders can reject recursive fs.watch. */
+/** Some workspace roots reject recursive fs.watch, especially Windows UNC paths. */
 const POLL_INTERVAL_MS = 2_000;
 
 interface ActiveWatcher {
@@ -65,16 +65,25 @@ function toForwardSlashes(path: string): string {
   return sep === '/' ? path : path.split(sep).join('/');
 }
 
-function shouldFallbackToPolling(err: unknown): boolean {
+function watchErrorCode(err: unknown): string | null {
   const code = (err as NodeJS.ErrnoException).code;
-  return code === 'EPERM' || code === 'EACCES' || code === 'EISDIR';
+  return typeof code === 'string' && code.length > 0 ? code : null;
 }
 
-function isWorkspaceUnavailableWatchError(err: unknown): boolean {
-  const code = (err as NodeJS.ErrnoException).code;
+function shouldFallbackToPolling(err: unknown): boolean {
+  const code = watchErrorCode(err);
+  return code !== null && !isWorkspaceUnavailableWatchErrorCode(code);
+}
+
+function isWorkspaceUnavailableWatchErrorCode(code: string): boolean {
   return code === 'ENOENT' || code === 'ENOTDIR';
 }
 
+function isWorkspaceUnavailableWatchError(err: unknown): boolean {
+  const code = watchErrorCode(err);
+  return code !== null && isWorkspaceUnavailableWatchErrorCode(code);
+}
+
 async function pollWorkspaceSignature(root: string): Promise<string> {
   const rows: string[] = [];