Make sidebar width proportional to window size and persist across sessions

CHANGELOG.md

@@ -9,6 +9,8 @@ Note that this project **does not** adhere to [Semantic Versioning](https://semv
 
 ## [Unreleased]
 
+- Sidebar width now persists across sessions and resizes proportionally to window width [#13402](https://github.com/JabRef/jabref/issues/13402)
+
 ### Added
 
 - We introduced a settings parameter to manage citations' relations local storage time-to-live with a default value set to 30 days. [#11189](https://github.com/JabRef/jabref/issues/11189)

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

@@ -213,6 +213,7 @@ public JabRefFrame(Stage mainStage,
         initKeyBindings();
         frameDndHandler.initDragAndDrop();
         initBindings();
+        initSidebarResizeListener();
     }
 
     private void initLayout() {
@@ -295,13 +296,31 @@ private void updateEditorPane() {
     }
 
     public void updateHorizontalDividerPosition() {
-        if (mainStage.isShowing() && !sidePane.getChildren().isEmpty()) {
-            horizontalSplit.setDividerPositions(preferences.getGuiPreferences().getHorizontalDividerPosition() / horizontalSplit.getWidth());
-            horizontalDividerSubscription = EasyBind.valueAt(horizontalSplit.getDividers(), 0)
-                                                    .mapObservable(SplitPane.Divider::positionProperty)
-                                                    .listenToValues((_, newValue) -> preferences.getGuiPreferences().setHorizontalDividerPosition(newValue.doubleValue()));
+    if (mainStage.isShowing() && !sidePane.getChildren().isEmpty()) {
+        double savedProportion = preferences.getGuiPreferences().getHorizontalDividerPosition();
+
+        // Fix: fallback to 0.2 if saved value is invalid
+        if (savedProportion <= 0.0 || savedProportion >= 1.0 || Double.isNaN(savedProportion)) {
+            savedProportion = 0.2;
+        }
+
+        horizontalSplit.setDividerPositions(savedProportion);
+
+        // Subscribe to save user-adjusted sidebar position
+        if (horizontalDividerSubscription != null) {
+            horizontalDividerSubscription.unsubscribe();
+        }
+
+        horizontalDividerSubscription = EasyBind.valueAt(horizontalSplit.getDividers(), 0)
+                .mapObservable(SplitPane.Divider::positionProperty)
+                .listenToValues((_, newValue) -> {
+                    double newPos = newValue.doubleValue();
+                    if (newPos > 0.0 && newPos < 1.0) {
+                        preferences.getGuiPreferences().setHorizontalDividerPosition(newPos);
+                    }
+                });
+            }
         }
-    }
 
     public void updateVerticalDividerPosition() {
         if (mainStage.isShowing() && panelMode.get() == PanelMode.MAIN_TABLE_AND_ENTRY_EDITOR) {
@@ -447,6 +466,14 @@ private void initBindings() {
         EasyBind.subscribe(preferences.getWorkspacePreferences().hideTabBarProperty(), _ -> updateTabBarVisible());
     }
 
+    private void initSidebarResizeListener() {
+        mainStage.widthProperty().addListener((obs, oldVal, newVal) -> {
+            if (!sidePane.getChildren().isEmpty()) {
+                updateHorizontalDividerPosition();
+            }
+        });
+    }
+
     private void updateTabBarVisible() {
         if (preferences.getWorkspacePreferences().shouldHideTabBar() && stateManager.getOpenDatabases().size() <= 1) {
             if (!tabbedPane.getStyleClass().contains("hide-tab-bar")) {