Height of the "header" (icon + search text box) depends on the icon size. Some application icons (e.g. Ark, digiKam) are smaller than others, if such application is selected, the height of the header is reduced. This produces rather annoying layout changes when the user scrolls through available applications using cursor keys. Expected behavior: height of the header is fixed and fits a "Normal" size icon.
Fixed in 53830d5.