Yes, @teh_g is correct, global moderators are special in that they will always have moderate privilege outside of the privileges page. Unfortunately it doesn't look as though you will be able to restrict gmods from their moderation privileges for this forum -- it's quite an odd use case!
@julian thanks for the hint- yes i resize it show up. but i still notice some issue - after i click one category from the category picker window, the chose category didn't show in the parent category field. after i refresh the page, it then show up. but not a big issue.
i have a suggestion --- instead of using a popup menu window for a list of all categories, maybe just use a auto-complete text box to allow user to type in the category name.... this will be much simple when we have many categories (i.e more than 30....).