Carefully rebind the $clean_event.

% bindkey "^I" complete-word
  % bi<TAB>

If we had not source the zsh-syntax-highlighting.zsh, it could
complete something. This is due to that `complete-word` will be
rebinded with 'zle -N $clean_event'.

Signed-off-by: Takeshi Banse <takebi@laafc.net>

zsh-syntax-highlighting.zsh

@@ -230,8 +230,14 @@ _zsh_highlight-install() {
           ;;
         .*)
           clean_event=$event[2,${#event}] # Remove the leading dot in the event name
+          case ${widgets[$clean_event]-} in
+            (completion|user):*)
+              ;;
+            *)
               eval "$clean_event() { builtin zle $event && _zsh_highlight-zle-buffer } ; zle -N $clean_event"
               ;;
+          esac
+          ;;
         *)
           ;;
       esac