Responsive navbar is 2px shorter when content is hidden.

Example: http://v4-alpha.getbootstrap.com/components/navbar/#nav, see first snippet.

At narrow screen navbar block has height 8 + 38 + 8 px (54px). At wide screen height become 8 + 40 + 8 px (56px). Reproducible in latest FF/Chromium, Ubuntu 14.04.

I think height should not change. Any quick-tips how to fix?