After updating to yakkety I’ve noticed a number of serious usability regressions with menus, especially the main menu (advanced menu).
Most irritating is that submenus do not stay open when moving the mouse towards them. In order to access a submenu you must move the mouse horizontally without mousing over an adjacent menu item. Previously, the submenu stays open if the mouse is moving diagonally toward it.
Secondly, clicking a menu item causes glitchy behaviour. The expected result is to open the submenu without delay, but in Ubuntu MATE 16.10 the submenu will appear then immediately disappear.
This seems to apply to all GTK3 menus, not just the MATE main menu. GTK2 menus, such as in The Gimp behave correctly.
Yes, I’ve noticed this too, with all GTK+3 apps and on other desktops
It was reported to the GNOME bugtracker in 2013, but there’s no response. (GNOME 3 apps don’t usually have tradition menubars, so it doesn’t matter much for GNOME.) Maybe now that other desktops are using GTK+3 someone can look closer at it.