I visited http://livingstyleguide.wmflabs.org/wiki/OOjs_UI/Widgets/Icons,_Indicators,_and_Labels and Blueprint's top bar wrapped to a second line and obscured the heading.
This screenshot shows the problem.
(The screenshot also shows a separate bug T103734: Blueprint skin doesn't display what you type in its search box using Firefox.)
Vector's top tabs can overflow and obscure the top line, but at least the unused parts of Vector's overflow don't obscure the first heading: