Apparently, with GTK3
gdk_drag_begin() accesses fields from the passed in event that aren't copyable by value -- whereas it worked with GTK2. So, one need to properly use
gdk_event_copy() instead of
ev = *event to copy the event.
Without this, starting a drag with a GTK3 Scintilla accesses freed memory.
Attached is the output from Valgrind when starting a drag under GTK3.