KMail has got a header above the list of mails that shows 'more' by default and opens the actual quick filter bar below on click. Is there a way to get rid of this 'more' header (IMHO it makes no sense to add a toolbar just to show or hide another; some kind of horizontal expander would have been the better choice)? And why didn't you integrate this show/don't show option into the configuration dialog?

I'm not sure it is possible, at least not easily.

If you didn't implement such a standard functionality I vote for doing it in 4.14 (or 5?). That means: remove the 'more' header and add 'show quick filter' to the options.

That "you" should be directed at the PIM developers rather than me. You should file a wishlist request on bugs.kde.org.