WIP (untested): implement idea from zsh-users#288 (comment)

Author: danielshahaf

zsh-syntax-highlighting.zsh

@@ -296,12 +296,15 @@ _zsh_highlight_bind_widgets()
       builtin) eval "_zsh_highlight_widget_${(q)cur_widget}() { _zsh_highlight_call_widget .${(q)cur_widget} -- \"\$@\" }"
                zle -N $cur_widget _zsh_highlight_widget_$cur_widget;;
 
+      *) 
       # Incomplete or nonexistent widget: Bind to z-sy-h directly.
-      '') _zsh_highlight_widget_${cur_widget}() { :; _zsh_highlight }
-          zle -N $cur_widget _zsh_highlight_widget_$cur_widget;;
-
+         if [[ $cur_widget == zle-* ]] && [[ -z $widgets[$cur_widget] ]]; then
+           _zsh_highlight_widget_${cur_widget}() { :; _zsh_highlight }
+           zle -N $cur_widget _zsh_highlight_widget_$cur_widget;;
+         else
       # Default: unhandled case.
-      *) print -r -- >&2 "zsh-syntax-highlighting: unhandled ZLE widget '$cur_widget'" ;;
+           print -r -- >&2 "zsh-syntax-highlighting: unhandled ZLE widget '$cur_widget'" ;;
+         fi
     esac
   done
 }