在GridBagLayout中的特定图块上移动时，JPanel消失了

Question

我有一个JLayeredPane和一个GridBagLayout布局，其中另一个JPanel在用户的键盘输入上移动。

一切正常，面板（称为圆点）移动正常，但是在Tile（表示右下角）的最后Grid处消失了。之后程序仍然运行，我可以正常将点面板移入和移出此Tile，但它不会出现在屏幕上。

这是我的GridPane持有Tile的代码：

public class GridPane extends JLayeredPane {
    public int rows;
    public int columns;

    private List<List<Tile>> tileGrid;
    private GridBagConstraints gbc = new GridBagConstraints();
    private KeyListener kl = new KeyListener() {

        @Override
        public void keyTyped(KeyEvent e) {

        }

        @Override
        public void keyReleased(KeyEvent e) {
            // TODO Auto-generated method stub

        }

        @Override
        public void keyPressed(KeyEvent e) {
                switch (e.getKeyCode()) {
                    case KeyEvent.VK_RIGHT:
                    if(dotTile.column < columns-1)
                    dotTile = tileGrid.get(dotTile.row).get(dotTile.column+1);
                        break;
                case KeyEvent.VK_LEFT:
                    if(dotTile.column != 0)
                    dotTile = tileGrid.get(dotTile.row).get(dotTile.column-1);
                        break;
                case KeyEvent.VK_DOWN:
                    if(dotTile.row < rows-1)
                    dotTile = tileGrid.get(dotTile.row+1).get(dotTile.column);
                        break;
                case KeyEvent.VK_UP:
                    if(dotTile.row != 0)
                    dotTile = tileGrid.get(dotTile.row-1).get(dotTile.column);
                        break;
                }
            //moving the dot
            remove(dot);
            gbc.fill = GridBagConstraints.NONE;
            gbc.anchor = GridBagConstraints.CENTER;
            gbc.gridx = dotTile.column;
            gbc.gridy = dotTile.row;
            add(dot, gbc, 1);
            revalidate();
            repaint();
        }
    };

    private JPanel dot = new JPanel();
    public Tile dotTile;

    public GridPane(int rows, int columns) {

        this.rows = rows;
        this.columns = columns;

        setLayout(new GridBagLayout());
        addKeyListener(kl);
        setFocusable(true);

        //fill array and JPanel with Tile
        tileGrid = new ArrayList<List<Tile>>(rows);
        for (int i = 0; i < rows; i++) {
            List<Tile> rowList = new ArrayList<Tile>(columns);
            for(int j=0; j < columns; j++) {
                Tile t = new Tile(randomColor(), i, j);
                rowList.add(t);
                gbc.fill = GridBagConstraints.HORIZONTAL;
                gbc.gridx = j;
                gbc.gridy = i;
                add(t, gbc, 0);
            }
            tileGrid.add(rowList);
        }

        dot.setPreferredSize(new Dimension(20, 20));
        dot.setBackground(Color.BLACK);

        //set initial position of dot
        dotTile = tileGrid.get(rows/2+1/2).get(columns/2+1/2);
        gbc.fill = GridBagConstraints.NONE;
        gbc.anchor = GridBagConstraints.CENTER;
        gbc.gridx = dotTile.column;
        gbc.gridy = dotTile.row;

        add(dot, gbc, 1);
        repaint();
    }

    private Random r = new Random();
    public Color randomColor() {
        return new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));

    }

    Color c = new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));
    private Color gradientColor() {
        c = c.darker();
        return c;
    }
}