Introduce SELECTION, which is said to be available on Linux

CHANGELOG.md

@@ -11,6 +11,7 @@ We refer to [GitHub issues](/~https://github.com/JabRef/jabref/issues) by using `#
 ## [Unreleased]
 
 ### Changed
+- When copying and pasting, the X11 selection is read first and also set. [#2389](/~https://github.com/JabRef/jabref/issues/2389).
 - When editing an article, the tab "Optional fields" now shows "ISSN"
 - When editing a book, the tab "Optional fields" now shows "ISBN"
 

src/main/java/net/sf/jabref/gui/ClipBoardManager.java

@@ -29,7 +29,7 @@ public class ClipBoardManager implements ClipboardOwner {
     private static final Log LOGGER = LogFactory.getLog(ClipBoardManager.class);
 
     private static final Clipboard CLIPBOARD = Toolkit.getDefaultToolkit().getSystemClipboard();
-    private static final Clipboard SELECTION = Toolkit.getDefaultToolkit().getSystemSelection();
+    private static final Optional<Clipboard> SELECTION = Optional.ofNullable(Toolkit.getDefaultToolkit().getSystemSelection());
 
     /**
      * Empty implementation of the ClipboardOwner interface.
@@ -46,7 +46,7 @@ public void lostOwnership(Clipboard aClipboard, Transferable aContents) {
     public void setClipboardContents(String aString) {
         StringSelection stringSelection = new StringSelection(aString);
         CLIPBOARD.setContents(stringSelection, this);
-        SELECTION.setContents(stringSelection, this);
+        SELECTION.ifPresent(s -> s.setContents(stringSelection, this));
 
     }
 
@@ -55,7 +55,7 @@ public void setClipboardContents(String aString) {
      */
     public void setTransferableClipboardContents(Transferable transferable){
         CLIPBOARD.setContents(transferable, this);
-        SELECTION.setContents(transferable, this);
+        SELECTION.ifPresent(s -> s.setContents(transferable, this));
     }
 
     /**
@@ -67,7 +67,10 @@ public void setTransferableClipboardContents(Transferable transferable){
     public String getClipboardContents() {
         String result = "";
         //odd: the Object param of getContents is not currently used
-        Transferable contents = SELECTION.getContents(null);
+        Transferable contents = null;
+        if (SELECTION.isPresent()) {
+            contents = SELECTION.get().getContents(null);
+        }
         if (contents == null) {
             contents = CLIPBOARD.getContents(null);
         }