Step to reproduce:
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1)Run the attached program.
2)Click on Vertical scrollbar of the textarea & type some characters untill you see a horizontal scrollbar. Observe that Horizontal scrollbar arrow buttons are not matching with the windows xp theme textarea arrow buttons. I have attached the screen shots of the described problem. If you see the same then the bug is reproduced.
3) moving the mouse pointer over the arrow buttons. The buttons remains the same. Click on the arrow buttons, now you can see the arrow buttons change to windows xp Theme textarea arrow buttons( matches with the native).

The same scenario can be repeated for Vertical scrollbar also.
1) Repeat step(1) .
2) Click on the horizontal scrollbar of the textarea, some character untill you see the vertical scrollbar. you can see the classic textarea arrow buttons.

I tested this with Mustang b90. The same problem can be seen in Tiger 1.5.0 /fcs/b64 & 1.4.2/fcs/b28 also. But in 1.4.2/fcs/b28 moving the mouse pointer over the arrow button changes the arrow button to match the native.