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.