Added scroll line method

richtextfx/src/main/java/org/fxmisc/richtext/GenericStyledArea.java

@@ -1393,7 +1393,7 @@ public void nextPage(SelectionPolicy selectionPolicy) {
      * @param pgCount the number of pages to page up/down.
      * <br>Negative numbers for paging up and positive for down.
      */
-    private void page(int pgCount, SelectionPolicy selectionPolicy)
+    private void page(double pgCount, SelectionPolicy selectionPolicy)
     {
         // Use underlying caret to get the same behaviour as navigating up/down a line where the x position is sticky
         Optional<Bounds> cb = caretSelectionBind.getUnderlyingCaret().getCaretBounds();
@@ -1410,6 +1410,25 @@ private void page(int pgCount, SelectionPolicy selectionPolicy)
             .filter( delta -> delta != 0.0 ).ifPresent( delta -> scrollYBy( delta ) ) ); 
     }
 
+    /**
+     * Scrolls nLines down (for positive numbers) or up (for negative numbers),
+     * while maintaining the caret's current position on screen.
+     * @param selectionPolicy
+     * @param nLines
+     */
+    public void scrollLines(SelectionPolicy selectionPolicy, int nLines) {
+        if (nLines == 0) return;
+
+        int direction = nLines < 0 ? -1 : +1;
+        double viewHeight = getViewportHeight() * direction;
+
+        for (int lines = nLines; lines != 0; lines -= direction)
+        {
+            getCaretBounds().map( caret -> (caret.getHeight()-2) / viewHeight )
+               .ifPresent(lineHeight -> page(lineHeight, selectionPolicy));
+        }
+    }
+
     @Override
     public void displaceCaret(int pos) {
         caretSelectionBind.displaceCaret(pos);