Re-assigning to Stephan, since he's in charge of the preview JS stuff.

It's necessary feature to view categories in page edit preview.
please set Priority to high

Please elaborate why this is necessary.

when writing article from zero this thing it's needed to view if entered categories exists. It's impossible to know perfectly wiki's category three.
Since this thing is missing, when i write different, more complex articles, i just not enter any category and thereupon i search them via HotCat.
But, a lot of users does not use gadgets like HotCat, and anonymous user also can't, so they just can introduce wrong titled categories without having possibility to check fast if it exists.

also displaying categories in preview can prevent saving mispelling typos in them, and, in generally it avoid multiple edits of page because of apparently minor things like incorrect or inexistent categories.

i was close to fill in new bug as enhancement feature request, but i found this bug via search and thus i wrote here.
thank you