In blender if you click a menu open in a header, and then change mouse to hover another of the top level menus, the clicked menu will just stay open, and closes after mouse has moved enough. The standard way menus work in most other software and GUI toolkits is to change the opened menu if user hover overs another menu. Maybe blender could also agree to the norm here, currently it's just a bit out of the line with everything without a good reason. Also menus usually close if the menu name is clicked, in blender nothing happens, or if some menu item is left active by the mouse it is executed. This is a bit confusing.
Blender behaving more standard in this case is absolutely fine, the way it works now is probably because no one tried to implement it or there was some difficulty trying to do it. Would you like to look into this, or is this something we should move to the todo list?
Also should add arrow key handling in the menus to the task (apparently I can't edit it because I'm not moderator and it's not assigned to me).
Left/right keys usually change the open top level menu, except if the motion opens a submenu.