Differences in margin (top and bottom) on dropdown-menus.

[vsn4ik]

• .dropdown-menu (margin-top: 2px - *1) and .dropup .dropdown-menu (margin-bottom: 1px - *2) not equal.
• .navbar-nav > li > .dropdown-menu (margin-top: 0px - *3) and .navbar-fixed-bottom .dropdown .dropdown-menu (margin-bottom: 1px - *2) not equal.

*1: https://github.com/twbs/bootstrap/blob/master/less/dropdowns.less#L38.
*2: https://github.com/twbs/bootstrap/blob/master/less/dropdowns.less#L193.
*3: https://github.com/twbs/bootstrap/blob/master/less/navbar.less#L329

[mdo]

The dropdowns and dropups should likely have the same value for top and bottom, respectively, but the navbar is purposely overridden to be attached to the navbar.