A comment on User Experience reminded me that Windows 98 keeps the All Programs submenus open when the mouse isn't on the menus, but if the mouse strays too far from the menus for long enough they all close.
I think that submenus also remain open if you move the mouse down to the next item and then across to the submenu if you do so quickly enough, but this may depend on a setting.
I have a suspicion that this behaved the same way as the Alt menus in Windows Forms programs.
How does Windows 98 determine when to open and close menus when accessed by the mouse?