Ctrl + Alt + Tab

Switches the focus between the panels and the desktop background. When you use these shortcut keys, a list of items that you can select is displayed. Release the keys to select an item. 

Ctrl + Alt + Esc

Switches the focus between the panels and the desktop background. Release the keys to select an item. 

Ctrl + F10

Opens the popup menu for the selected panel. 

Tab

Switches the focus between objects on a panel. 

Return

Chooses the selected panel object or menu item. 

Shift + F10

Opens the popup menu for the selected panel object. 

Arrow keys 

Moves the focus between items in a menu. Moves the focus between interface items in an applet also. 

Esc

Closes an open menu. 

F10

Opens the Applications menu from the Menu Bar, if the Menu Bar is in a panel.