Perhaps it would be worth to add a simple "Copy" in addition to "Copy preview" to right-click menu of preview pane. Sometimes it is convenient to copy just a part of preview (selected beforehand), and to use right-click menu for that, as in many other programs. But in JabRef (version 5.1) right-click menu only allows one to copy the whole preview. By the way, Ctrl-C still works, and thus is a workaround.