GTK+ now includes an OpenGL rendering widget. To support GL on various platforms, GTK+ uses libepoxy.
GTK+ no longer uses gtk-update-icon-cache during its build. The --enable-gtk2-dependency configure option has been removed.
The introspection annotations for the x and y parameters of GtkMenuPositionFunc have been corrected from 'out' to 'inout'. If you are using such a function from language-bindings, this may require adjustments.
The lookup order for actions that are activated via keyboard accelerators has been changed to start at the currently focused widget. If your application is making use fo nested action groups via gtk_widget_insert_action_group, you may want to check that this change does not upset your accelerators.
The GtkScrollable interface has gained a new vfunc, get_border, that is used to position overshoot and undershoot indications that are drawn over the content by GtkScrolledWindow. Unless your scrollable has non-scrolling parts similar to treeview headers, there is no need to implement this vfunc.
The GtkSearchEntry widget has gained a number of new signals that are emitted when certain key sequences are seen. In particular, it now handles the Escape key and emits ::stop-search. Applications that expect to handle Escape themselves will need to be updated.