Menus are no longer opened as "sticky" by default. A sticky menu is when

it stays open even when the mouse moves away, unless clicking outside of

the menu or triggering another menu to open. This sort of menu makes

sense for some kinds of UI elements, but can be a nuisance when hovering

over simple dropdowns of options (like we have in the review request

and top bar actions in Review Board).
Now a menu will close when moving the mouse outside of the menu or its

controller by default. Passing sticky: true when opening will now place

the menu in sticky mode (whether or not the menu is currently open).
MenuButton has been updated to open in sticky mode when using a

keyboard shortcut or a direct click on the dropdown button. Otherwise,

it opens in a non-sticky mode.

Unit tests pass.
Tested that menus in Review Board weren't sticky by default. Clicking the

dropdown handle on a MenuButton did result in a sticky menu.