My previous default behaviour of appending SQL history when a statement is double clicked has been replaced by a replace.
I have dim and distant memories of setting this default behaviour in a previous version of SQL Developer, but can't find/recall a way of doing it now.
If this is set in the preferences, then my preference has not been carried forward in 4 EA3.
I know I can click the icon to the left of the SQL statement, but that is a more cumbersome <click>, <move mouse>, <click>, rather than the previous <double click> that worked as recently as 4 EA2.