lib/fs: Consider win83 for root path as well when watching (ref #5706) (#5709)

lib/fs/basicfs_windows.go

@@ -222,3 +222,28 @@ func evalSymlinks(in string) (string, error) {
 	}
 	return longFilenameSupport(out), nil
 }
+
+// watchPaths adjust the folder root for use with the notify backend and the
+// corresponding absolute path to be passed to notify to watch name.
+func (f *BasicFilesystem) watchPaths(name string) (string, string, error) {
+	root, err := evalSymlinks(f.root)
+	if err != nil {
+		return "", "", err
+	}
+
+	// Remove `\\?\` prefix if the path is just a drive letter as a dirty
+	// fix for https://github.com/syncthing/syncthing/issues/5578
+	if filepath.Clean(name) == "." && len(root) <= 7 && len(root) > 4 && root[:4] == `\\?\` {
+		root = root[4:]
+	}
+
+	absName, err := rooted(name, root)
+	if err != nil {
+		return "", "", err
+	}
+
+	root = f.resolveWin83(root)
+	absName = f.resolveWin83(absName)
+
+	return filepath.Join(absName, "..."), root, nil
+}