GtkWidget had a hack where if opacity is 0.999 we set up an opacity group when rendering the widget. This is no longer needed in 3.10, and GtkStack doesn't use it anymore. It has been removed in 3.12. GdStack is using it, so applications should be ported from GdStack to GtkStack in 3.12.
GtkHeaderBar in 3.10 was not ordering its pack-end children in the right way. This has been fixed in 3.12. Applications which pack multiple widgets at the end of a headerbar will have to be updated.
gtk_text_view_add_child_in_window has changed behaviour a bit. It now always positions the child in buffer coordinates, where it used to inconsistently scroll with the buffer but then go reposition to a window-relative position on redraw.
A number of container widgets have been made more compliant with the uniform CSS rendering model by making them render backgrounds and borders. This may require some adjustments in applications that were making assumptions about containers never rendering backgrounds.