Clearly the most improved module in 3.12 cycle was GTK+, and we particularly have to thank Matthias Clasen for that. From GTK what I love the most is Popovers.
Allan Day explains in his blog post “Popovers & You“, the cases we should use them. I will simplify this and I will just say “EVERYWHERE!”.
Traditional menus are dead from Web-Design and are dead from GNOME too. Popovers came too late in the 3.12 cycle, so many applications didn’t have the time to use them, or others didn’t have the time to fully use them. In my very own speculation Popovers will replace almost every other menu in GNOME 3.14, and that is coming straight from the latest designs.
But how Popovers treat sub-menus? A sub-menu (menu-child) usually opens on the left/right by hovering the parent item. This way is fast (often leads to mistakes!), but popovers can’t do that.
Totem (and of course every other Gnome app) uses the Firefox way with left/right sliding the whole content. You may think this is kinda slower from a traditional menus but..
..popovers can combine many functions (filter input, check-boxes etc) so in the big picture Popovers will make things faster. Also we have to take into consideration that hover isn’t really working on touch screens.
In my opinion Submenus Couldn’t Be Better, aside the fact of badly re-sizing the popover box as seen on the screencasting.