There will be a hover style for the CC button that will override any other styling that might be on the CC button.
When the user clicks on the CC button
The menu will hide when any one of the following happens:
The currently selected item in the menu will have its own style. The menu items will have a hover style different than the 'currently selected' style.
The styling of the CC button will change to reflect whether or not a caption is currently selected. The "captions are on" style is different than the "menu is currently showing" style.
There will be a focus style for the CC button that will be different than the hover style.
Tabbing to the CC button will focus it but not show the menu. Enter or space will show the menu.
Once the menu is showing, arrow keys will move 'focus' within the menu items. This action alone will NOT actually 'activate' an item in the menu.
Items selected with the arrow keys will be styled differently than the currently active selection. James is still deciding whether or not this style will be different than the hover style.
While the menu is showing, enter or space will hide the menu. If the arrow keys had been used to change the selection, the new selection will be activated with the enter or space.
If the user has used the arrow keys to select something but NOT yet pressed enter or space, tabbing away from the CC button will hide the menu WITHOUT activating the selected menu item.