Scroll events have been separated from button events, and smooth scrolling has been added with a separate event mask. Widgets now need to have either GDK_SCROLL_MASK or GDK_SMOOTH_SCROLL_MASK in their event mask to receive scroll events. In addition, the GdkScrollDirection enumeration has gained a new member, GDK_SCROLL_SMOOTH, so switch statements will have to be amended to cover this case.
GTK+ now uses <Primary> instead of <Control> in keyboard accelerators, for improved cross-platform handling. This should not affect applications, unless they parse or create these accelerator manually.
The tacit assumption that the Alt key corresponds to the MOD1 modifier under X11 is now a hard requirement.
The beagle search backend for the file chooser has been dropped. Tracker is the only supported search backend on Linux now.
GtkNotebook has been changed to destroy its action widgets when it gets destroyed itself. If your application is using action widgets in notebooks, you may have to adjust your code to take this into account.
GtkApplication no longer uses the gtk_ mainloop wrappers, so
it is no longer possible to use
gtk_main_quit() to stop it.
The -uninstalled variants of the pkg-config files have been dropped.
Excessive dependencies have been culled from Requires: lines in .pc files. Dependent modules may have to declare dependencies that there were getting 'for free' in the past.