scrollbars in JTextArea doesn't work properly

pagedown/pageup through scrollbars (by clicking on the trough under the scrollbar) in JTextArea doesn't work.

To reproduce,
1. run swing demo program Notepad that use JTextArea
2. fill in the textarea more than a page
3. click on the trough under the scrollbar for pagedown
4. result nothing happen.
5. same problem with horizontal one.