Show a welcome screen if no database is open

src/main/java/org/jabref/gui/Base.css

@@ -1734,9 +1734,41 @@ We want to have a look that matches our icons in the tool-bar */
     -fx-effect: dropshadow(three-pass-box, rgba(0, 0, 0, 0.6), 8, 0.0, 0, 0);
 }
 
+.welcome-label {
+    -fx-font-size: 40px;
+    -fx-text-fill: -jr-theme-text;
+    -fx-font-family: "Arial";
+}
+
+.description-label {
+    -fx-font-size: 25px;
+    -fx-font-family: "Arial";
+}
+
+.header-label {
+    -fx-font-size: 20px;
+    -fx-font-weight: bold;
+    -fx-font-family: "Arial";
+}
+
+.no-recent-label {
+    -fx-font-size: 18px;
+    -fx-text-fill: -jr-theme;
+    -fx-font-family: "Arial";
+}
+
+.welcome-hyperlink {
+    -fx-font-size: 18px;
+    -fx-text-fill: -jr-theme;
+}
+
+.welcome-hyperlink:visited {
+    -fx-text-fill: -jr-theme;
+}
+
 /* AboutDialog */
 #aboutDialog .about-heading {
-    -fx-font-size: 30;
+    -fx-font-size: 31;
     -fx-text-fill: -jr-theme;
 }
 

src/main/java/org/jabref/gui/frame/JabRefFrame.java

@@ -18,12 +18,14 @@
 import javafx.collections.transformation.FilteredList;
 import javafx.event.Event;
 import javafx.scene.control.ContextMenu;
+import javafx.scene.control.MenuItem;
 import javafx.scene.control.SeparatorMenuItem;
 import javafx.scene.control.SplitPane;
 import javafx.scene.control.Tab;
 import javafx.scene.control.TabPane;
 import javafx.scene.input.KeyEvent;
 import javafx.scene.layout.BorderPane;
+import javafx.scene.layout.StackPane;
 import javafx.scene.layout.VBox;
 import javafx.stage.Stage;
 
@@ -49,6 +51,7 @@
 import org.jabref.gui.sidepane.SidePaneType;
 import org.jabref.gui.undo.CountingUndoManager;
 import org.jabref.gui.util.BindingsHelper;
+import org.jabref.gui.util.WelcomePage;
 import org.jabref.logic.UiCommand;
 import org.jabref.logic.ai.AiService;
 import org.jabref.logic.journals.JournalAbbreviationRepository;
@@ -104,6 +107,9 @@ public class JabRefFrame extends BorderPane implements LibraryTabContainer, UiMe
 
     private Subscription dividerSubscription;
 
+    private final WelcomePage welcomePage;
+    private final StackPane contentPane = new StackPane();
+
     public JabRefFrame(Stage mainStage,
                        DialogService dialogService,
                        FileUpdateMonitor fileUpdateMonitor,
@@ -125,6 +131,30 @@ public JabRefFrame(Stage mainStage,
         this.clipBoardManager = clipBoardManager;
         this.taskExecutor = taskExecutor;
 
+        this.fileHistory = new FileHistoryMenu(
+                this.preferences.getLastFilesOpenedPreferences().getFileHistory(),
+                dialogService,
+                getOpenDatabaseAction());
+
+        this.fileHistory.getItems().addListener((ListChangeListener<MenuItem>) change -> {
+            boolean hasRecentFiles = !fileHistory.getItems().isEmpty();
+            fileHistory.setDisable(!hasRecentFiles);
+        });
+
+        this.welcomePage = new WelcomePage(
+                this,
+                preferences,
+                aiService,
+                dialogService,
+                stateManager,
+                fileUpdateMonitor,
+                entryTypesManager,
+                undoManager,
+                clipBoardManager,
+                taskExecutor,
+                fileHistory
+        );
+
         setId("frame");
 
         // Create components
@@ -174,10 +204,6 @@ public JabRefFrame(Stage mainStage,
                 this.preferences,
                 taskExecutor);
 
-        this.fileHistory = new FileHistoryMenu(
-                this.preferences.getLastFilesOpenedPreferences().getFileHistory(),
-                dialogService,
-                getOpenDatabaseAction());
         this.setOnKeyTyped(key -> {
             if (this.fileHistory.isShowing()) {
                 if (this.fileHistory.openFileByKey(key)) {
@@ -191,6 +217,8 @@ public JabRefFrame(Stage mainStage,
         frameDndHandler.initDragAndDrop();
         initBindings();
         initTabBarManager();
+        bindDatabaseChanges();
+        updateContent();
     }
 
     private void initLayout() {
@@ -230,11 +258,27 @@ private void initLayout() {
         setTop(head);
 
         splitPane.getItems().addAll(tabbedPane);
+        contentPane.getChildren().addAll(welcomePage, splitPane);
         SplitPane.setResizableWithParent(sidePane, false);
         sidePane.widthProperty().addListener(c -> updateSidePane());
         sidePane.getChildren().addListener((InvalidationListener) c -> updateSidePane());
         updateSidePane();
-        setCenter(splitPane);
+        setCenter(contentPane);
+        updateSidePane();
+        updateContent();
+    }
+
+    private void updateContent() {
+        boolean hasOpenDatabases = !stateManager.getOpenDatabases().isEmpty();
+        welcomePage.setVisible(!hasOpenDatabases);
+        splitPane.setVisible(hasOpenDatabases);
+
+        boolean hasRecentFiles = !fileHistory.getItems().isEmpty();
+        fileHistory.setDisable(!hasRecentFiles);
+    }
+
+    private void bindDatabaseChanges() {
+        stateManager.getOpenDatabases().addListener((InvalidationListener) obs -> Platform.runLater(this::updateContent));
     }
 
     private void updateSidePane() {

src/main/java/org/jabref/gui/util/WelcomePage.java

@@ -0,0 +1,124 @@
+package org.jabref.gui.util;
+
+import javafx.collections.ListChangeListener;
+import javafx.geometry.Pos;
+import javafx.scene.Node;
+import javafx.scene.control.Hyperlink;
+import javafx.scene.control.Label;
+import javafx.scene.control.MenuItem;
+import javafx.scene.layout.HBox;
+import javafx.scene.layout.VBox;
+
+import org.jabref.gui.ClipBoardManager;
+import org.jabref.gui.DialogService;
+import org.jabref.gui.StateManager;
+import org.jabref.gui.frame.FileHistoryMenu;
+import org.jabref.gui.frame.JabRefFrame;
+import org.jabref.gui.importer.NewDatabaseAction;
+import org.jabref.gui.importer.actions.OpenDatabaseAction;
+import org.jabref.gui.preferences.GuiPreferences;
+import org.jabref.gui.undo.CountingUndoManager;
+import org.jabref.logic.ai.AiService;
+import org.jabref.logic.l10n.Localization;
+import org.jabref.logic.util.TaskExecutor;
+import org.jabref.model.entry.BibEntryTypesManager;
+import org.jabref.model.util.FileUpdateMonitor;
+
+public class WelcomePage extends VBox {
+
+    private final VBox recentLibrariesBox;
+
+    public WelcomePage(JabRefFrame frame,
+                       GuiPreferences preferences,
+                       AiService aiService,
+                       DialogService dialogService,
+                       StateManager stateManager,
+                       FileUpdateMonitor fileUpdateMonitor,
+                       BibEntryTypesManager entryTypesManager,
+                       CountingUndoManager undoManager,
+                       ClipBoardManager clipBoardManager,
+                       TaskExecutor taskExecutor,
+                       FileHistoryMenu fileHistoryMenu) {
+
+        setAlignment(Pos.CENTER);
+        setSpacing(10);
+
+        this.recentLibrariesBox = new VBox(5);
+
+        VBox welcomeBox = createWelcomeBox();
+        VBox startBox = createStartBox(frame, preferences, aiService, dialogService, stateManager, fileUpdateMonitor, entryTypesManager, undoManager, clipBoardManager, taskExecutor);
+        VBox recentBox = createRecentBox(fileHistoryMenu);
+
+        VBox container = new VBox(20, welcomeBox, startBox, recentBox);
+        container.setAlignment(Pos.CENTER);
+
+        HBox mainContainer = new HBox(20, container);
+        mainContainer.setAlignment(Pos.CENTER);
+
+        getChildren().add(mainContainer);
+    }
+
+    private VBox createWelcomeBox() {
+        Label welcomeLabel = new Label(Localization.lang("Welcome to JabRef"));
+        welcomeLabel.getStyleClass().add("welcome-label");
+
+        Label descriptionLabel = new Label(Localization.lang("Stay on top of your Literature"));
+        descriptionLabel.getStyleClass().add("description-label");
+
+        return createBox(Pos.TOP_LEFT, welcomeLabel, descriptionLabel);
+    }
+
+    private VBox createStartBox(JabRefFrame frame, GuiPreferences preferences, AiService aiService, DialogService dialogService, StateManager stateManager, FileUpdateMonitor fileUpdateMonitor, BibEntryTypesManager entryTypesManager, CountingUndoManager undoManager, ClipBoardManager clipBoardManager, TaskExecutor taskExecutor) {
+        Label startLabel = new Label(Localization.lang("Start"));
+        startLabel.getStyleClass().add("header-label");
+
+        Hyperlink newLibrary = new Hyperlink(Localization.lang("New Library"));
+        newLibrary.getStyleClass().add("welcome-hyperlink");
+        newLibrary.setOnAction(e -> new NewDatabaseAction(frame, preferences).execute());
+
+        Hyperlink openLibrary = new Hyperlink(Localization.lang("Open Library"));
+        openLibrary.getStyleClass().add("welcome-hyperlink");
+        openLibrary.setOnAction(e -> new OpenDatabaseAction(frame, preferences, aiService, dialogService,
+                stateManager, fileUpdateMonitor, entryTypesManager, undoManager, clipBoardManager,
+                taskExecutor).execute());
+
+        return createBox(Pos.TOP_LEFT, startLabel, newLibrary, openLibrary);
+    }
+
+    private VBox createRecentBox(FileHistoryMenu fileHistoryMenu) {
+        Label recentLabel = new Label(Localization.lang("Recent"));
+        recentLabel.getStyleClass().add("header-label");
+
+        recentLibrariesBox.setAlignment(Pos.TOP_LEFT);
+        updateRecentLibraries(fileHistoryMenu);
+
+        fileHistoryMenu.getItems().addListener((ListChangeListener<MenuItem>) change -> updateRecentLibraries(fileHistoryMenu));
+
+        return createBox(Pos.TOP_LEFT, recentLabel, recentLibrariesBox);
+    }
+
+    private void updateRecentLibraries(FileHistoryMenu fileHistoryMenu) {
+        recentLibrariesBox.getChildren().clear();
+
+        if (fileHistoryMenu.getItems().isEmpty()) {
+            Label noRecentLibrariesLabel = new Label(Localization.lang("No Recent Libraries"));
+            noRecentLibrariesLabel.getStyleClass().add("no-recent-label");
+            recentLibrariesBox.getChildren().add(noRecentLibrariesLabel);
+        } else {
+            for (MenuItem item : fileHistoryMenu.getItems()) {
+                String filePath = item.getText();
+                Hyperlink recentLibraryLink = new Hyperlink(filePath);
+                recentLibraryLink.getStyleClass().add("welcome-hyperlink");
+                recentLibraryLink.setOnAction(item.getOnAction());
+                recentLibrariesBox.getChildren().add(recentLibraryLink);
+            }
+        }
+    }
+
+    private VBox createBox(Pos alignment, Node... nodes) {
+        VBox box = new VBox(5);
+        box.setAlignment(alignment);
+        box.getChildren().addAll(nodes);
+        return box;
+    }
+}

src/main/resources/l10n/JabRef_en.properties

@@ -2832,3 +2832,11 @@ Include=Include
 Exclude=Exclude
 Include\ or\ exclude\ cross-referenced\ entries=Include or exclude cross-referenced entries
 Would\ you\ like\ to\ include\ cross-reference\ entries\ in\ the\ current\ operation?=Would you like to include cross-reference entries in the current operation?
+
+New\ Library=New Library
+No\ Recent\ Libraries=No Recent Libraries
+Open\ Library=Open Library
+Recent=Recent
+Start=Start
+Stay\ on\ top\ of\ your\ Literature=Stay on top of your Literature
+Welcome\ to\ JabRef=Welcome to JabRef