This page lists some notable API/ABI breaks.
Probably not all were listed, but this is provided in hope that with help of this page migrating from gtksourceviewmm-2.0 to 3.0 won't be a cause of headaches.
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Stuff marked as deprecated was removed of course. 
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Removed Source prefix from all types, SOURCE_ prefix from all enum value names and source prefix from all file names. There is a helper script called source_truncate.pl in git repository in codegen directory to aid programmer in boring task of renaming everything. 
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"source-mark-updated" signal in Gsv::Buffer now takes Gtk::TextMark instead of Mark. 
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Vfuncs in Gsv::CompletionProposal, Gsv::CompletionProvider and Gsv::UndoManager are now private. They shouldn't be called directly anyway - there are respective methods for this. 
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Some methods now takes std::string instead of Glib::ustring. These are: 
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Gsv::Mark's constructor with category and name has parameter order the same as C gtk_source_mark_new(). Previously parameter order was reversed, because name parameter could be omitted - it has a default value of empty string, which was interpreted as creating anonymous Gsv::Mark. Now there is separate constructor for creating anonymous SourceMarks. If there was:  Now should be:  If there was:  Nothing needs to be changed. If there was:  Now should be:   
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If there is a code that somehow depended on Gsv::StyleScheme being a Glib::Interface, then this code is wrong, because it should always be a Glib::Object. 
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SourceIter is gone - all its features are now supported by Gtk::TextIter.  
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Gsv::Completion is now a Glib::Object, not Gtk::Object. So it should be used as Glib::RefPtr<Gsv::Completion> instead of plain Gsv::Completion. 
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Namespace gtksourceview was renamed to Gsv. If there is a lot of code using gtksourceview namespace then a workaround would be creating a backward compatibility header and including it a place included by affected sources. A header could contain a lines like that: #ifndef GTKSOURCEVIEWMM_COMPAT_H #define GTKSOURCEVIEWMM_COMPAT_H namespace gtksourceview =  Gsv; 
 #endif // GTKSOURCEVIEWMM_COMPAT_H 
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Replaced Glib::Handle<> with std::vector<> - that means that passing std::list<> or std::deque<> is now not supported. Affected methods are: 
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Gsv::Markup now has operator const void instead of operator bool, so implicit casts to int are avoided.