The GtkSourceEncoding class has changed.

The GtkSourceView class no longer implements the GtkBuildable interface. As a result, it is no longer possible to access the completion object with the internal-child "completion" in a UI definition. You need to do it programmatically instead.

The GtkSourceCompletion class no longer implements the GtkBuildable interface. As a result, it is no longer possible to access the info window object with the internal-child "info_window" in a UI definition. You need to write code instead and call the gtk_source_completion_get_info_window() function.

All the GtkSourceCompletionItem properties have been removed. You need to rely on the setter and getter functions from that class or from the GtkSourceCompletionProposal interface. To be notified of changes, connect to the GtkSourceCompletionProposal::changed signal.