• Fixed side pane width not persisting after JabRef restart
• Removed erroneous division of normalized divider position
• Horizontal divider now restored using normalized position directly
• Updated CHANGELOG with fix description

Remove incorrect divider position normalization

• Removed erroneous division of getHorizontalDividerPosition() by horizontalSplit.getWidth()
• Normalized divider position is now applied directly to setDividerPositions()
• Aligns horizontal divider restoration with existing vertical divider logic

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

Document side pane width fix

• Added entry documenting the fix for side pane width persistence issue
• References issue #8907 with link to GitHub issue

CHANGELOG.md

Description

updateHorizontalDividerPosition restores the persisted divider position without validating it, so
users with previously-corrupted stored values (e.g., near-zero) will still get a collapsed side pane
after upgrading until they manually resize it again.

Code

jabgui/src/main/java/org/jabref/gui/frame/JabRefFrame.java[R307-312]

    public void updateHorizontalDividerPosition() {
        if (mainStage.isShowing() && !sidePane.getChildren().isEmpty()) {
-            horizontalSplit.setDividerPositions(preferences.getGuiPreferences()
-                                                           .getHorizontalDividerPosition() / horizontalSplit.getWidth());
+            horizontalSplit.setDividerPositions(preferences.getGuiPreferences().getHorizontalDividerPosition());
            horizontalDividerSubscription = EasyBind.valueAt(horizontalSplit.getDividers(), 0)
                                                    .mapObservable(SplitPane.Divider::positionProperty)
                                                    .listenToValues((_, newValue) ->

Evidence

The horizontal divider restore now directly applies the stored preference value via
setDividerPositions(...), and the same method subscribes to divider position changes and persists
them. If the stored value is already out-of-range or extremely small, there is no fallback to a
default, so the UI will restore that bad value indefinitely.

jabgui/src/main/java/org/jabref/gui/frame/JabRefFrame.java[307-315]
jabgui/src/main/java/org/jabref/gui/CoreGuiPreferences.java[35-45]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

### Issue description
`JabRefFrame#updateHorizontalDividerPosition` restores the horizontal split divider position from preferences without any validation. If the stored preference value is corrupted (e.g., extremely small or not finite), the side pane will remain effectively collapsed until the user manually resizes.

### Issue Context
- Divider positions are normalized (0..1).
- There is a reasonable default (0.15) already defined in `CoreGuiPreferences`.

### Fix Focus Areas
- jabgui/src/main/java/org/jabref/gui/frame/JabRefFrame.java[307-315]
- jabgui/src/main/java/org/jabref/gui/CoreGuiPreferences.java[35-45]

### Suggested fix
- Before calling `horizontalSplit.setDividerPositions(...)`, validate the value:
 - if `Double.isNaN(value)` / `Double.isInfinite(value)` / `value <= 0.01` / `value >= 0.99` (thresholds as appropriate), fall back to `CoreGuiPreferences.getDefault().getHorizontalDividerPosition()`.
 - Optionally write the corrected value back into preferences so it self-heals.

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