/**
     * Stabilize (do not allow scrolling) the JViewport displaying
     * the indicated JEditorPane.
     * This is typically used when the underlying document may change while
     * being edited in another view. The {@link #stop} method is used to release
     * the listeners and so unfreeze the viewport.
     * <p>This is a one shot class. The editor is expected to be good to go.
     * Only document changes are listened to. The first char of the top line is
     * pinned to the upper left corner. If needed, this could be extended
     * to pin the horizontal position as well.
     */
    public static class FreezeViewport implements DocumentListener {
        private JEditorPane ep;
        private JViewport vp;
        private Document doc;
        private Position pos;
        private int topLine;
        private int nLine;
        
        public FreezeViewport(JEditorPane ep) {
            try {
                this.ep = ep;
                doc = ep.getDocument();
                if(doc == null)
                    return;
                vp = (JViewport)ep.getParent(); // may throw class cast, its ok
                Element root = doc.getDefaultRootElement();
                nLine = root.getElementCount();
                
                // Get the offset of the first displayed char in the top line
                Point pt = vp.getViewPosition();
                int offset = ep.viewToModel(pt);
                
                // Determine the line number of the top displayed line
                topLine = root.getElementIndex(offset);
                
                // Note. offset may not be first char, due to horiz scroll
                // make offset the first char of the line
                offset = root.getElement(topLine).getStartOffset();
                
                // Get marker to offset in the document
                pos = doc.createPosition(offset);
                doc.addDocumentListener(this);
            } catch (Exception ex) {
                // Note: did not start listener
            }
        }
    
        public void stop() {
            doc.removeDocumentListener(this);
        }
    
        private void track(DocumentEvent e) {
            // Might be able to use info from DocumentEvent to optimize
            try {
                Element root = doc.getDefaultRootElement();
                int newNumLine = root.getElementCount();
                // return if line count unchanged or changed after our mark
                if(nLine == newNumLine || e.getOffset() > pos.getOffset())
                    return;
                nLine = newNumLine;
                
                int newTopLine = root.getElementIndex(pos.getOffset());
                if(topLine == newTopLine)
                    return;
                topLine = newTopLine;
                
                // make a move
                int offset = root.getElement(topLine).getStartOffset();
                Point pt = ep.modelToView(offset).getLocation();
                pt.translate(-pt.x, 0); // x <-- 0, leave a few pixels to left
                vp.setViewPosition(pt);
            } catch(Exception ex) {
                stop();
            }
            return;
        }

        public void insertUpdate(DocumentEvent e) { track(e); }

        public void removeUpdate(DocumentEvent e) { track(e); }

        public void changedUpdate(DocumentEvent e) { }
    }