Currently, when I open the main menu and I hover the mouse over it, sub menus are selected (get open). When you go down to a sub/submenu (I tend to arrange them heirarchially) and unintentionally exit the submenu area all your selections are gone and you have to start all over. Back when I did SUN SOLARIS we could configure the menu to open only on click so we didn't loose the selection on such slides. I think this feature, once adopted by any of the window managers products, will be significant and will eventually penetrate the "competitors".
Talking to Avi Harel, Chair of the Technical Committee (TC) for Usability, he quoted me a standard describing such menu behaviour: "Guidance on World Wide Web user interfaces ISO 9241-151;2008 The use of expandable navigation menus is inconvenient when the number of total links (even if hidden) is high. In this case, users of non-visual and non-standard user agents will directly access all of the links, which may generate a cognitive overload. When dealing with a large number of links, a levelled navigation scheme is preferred"
If you want this feature, you'll have to bug the gtk+ toolkit guys, not us.
That will be great. This way also other menu systems will comply with this standard.