fix visual snap of side pane on startup

CHANGELOG.md

@@ -11,6 +11,7 @@ Note that this project **does not** adhere to [Semantic Versioning](https://semv
 
 ### Added
 
+- We fixed a glitch with the sidepane divider position on startup. [#15394](https://github.com/JabRef/jabref/issues/15394)
 - We added a label to the Group dropdown in the Import Dialog. [#15567](https://github.com/JabRef/jabref/issues/15567)
 - We added a related work text extractor, which finds and inserts the related work text into bib entries from references in the texts. [#9840](https://github.com/JabRef/jabref/issues/9840)
 - We added a hover button on group rows to quickly add a new group or subgroup. [#12289](https://github.com/JabRef/jabref/issues/12289)

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

@@ -284,8 +284,12 @@ private void updateSidePane() {
             horizontalSplit.getItems().remove(sidePane);
         } else {
             if (!horizontalSplit.getItems().contains(sidePane)) {
+                horizontalSplit.setVisible(false);
                 horizontalSplit.getItems().addFirst(sidePane);
-                updateHorizontalDividerPosition();
+                Platform.runLater(() -> {
+                    updateHorizontalDividerPosition();
+                    horizontalSplit.setVisible(true);
+                });
             }
         }
     }