Initializing EntryEditor Tabs on focus (#3331)

* Setting up the tab when it is initialized. All tabs that are not visible (end from FieldsEditor) will cost no memory or CPU

* Added entry to changelog

CHANGELOG.md

@@ -23,6 +23,8 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
 - We now set the WM_CLASS of the UI to org-jabref-JabRefMain to allow certain Un*x window managers to properly identify its windows
 - We changed the default paths for the OpenOffice/LibreOffice binaries to the default path for LibreOffice
 - We no longer create a new entry editor when selecting a new entry to increase performance. [#3187](https://github.com/JabRef/jabref/pull/3187)
+- We increased performance and decreased the memory footprint of the entry editor drastically. [#3331](https://github.com/JabRef/jabref/pull/3331)
+
 
 ### Fixed
  - We fixed the translation of \textendash in the entry preview [#3307](https://github.com/JabRef/jabref/issues/3307)

src/main/java/org/jabref/gui/entryeditor/FieldsEditorTab.java

@@ -39,19 +39,21 @@
  */
 class FieldsEditorTab extends EntryEditorTab {
 
-    private final Region panel;
     private final List<String> fields;
     private final EntryEditor parent;
     private final Map<String, FieldEditorFX> editors = new LinkedHashMap<>();
     private final JabRefFrame frame;
     private final BasePanel basePanel;
     private final BibEntry entry;
+    private final boolean isCompressed;
+    private Region panel;
     private FieldEditorFX activeField;
 
+
     public FieldsEditorTab(JabRefFrame frame, BasePanel basePanel, List<String> fields, EntryEditor parent, boolean addKeyField, boolean compressed, BibEntry entry) {
         this.entry = Objects.requireNonNull(entry);
         this.fields = new ArrayList<>(Objects.requireNonNull(fields));
-
+        this.isCompressed = compressed;
         // Add the edit field for Bibtex-key.
         if (addKeyField) {
             this.fields.add(BibEntry.KEY_FIELD);
@@ -61,8 +63,6 @@ public FieldsEditorTab(JabRefFrame frame, BasePanel basePanel, List<String> fiel
         this.frame = frame;
         this.basePanel = basePanel;
 
-        panel = setupPanel(frame, basePanel, compressed);
-
         // The following line makes sure focus cycles inside tab instead of being lost to other parts of the frame:
         //panel.setFocusCycleRoot(true);
     }
@@ -305,6 +305,7 @@ public void requestFocus() {
 
     @Override
     protected void initialize() {
+        panel = setupPanel(frame, basePanel, isCompressed);
         setContent(panel);
     }
 }